"I was motivated by the technology, and it's so good for the environment," Jim Alden said of his new Toyota Prius, a hybrid car.

"It's always great to go 500 miles on less than $20 in gas," said Jim Alden of Hollywood, who recently sold his 1994 Honda Accord and bought a $22,000 silver 2001 Toyota (news/quote) Prius, one of two hybrid gas-electric vehicles on the market, along with the Honda Insight.

Mr. Alden, 39, had paid more than $2 a gallon for gasoline earlier this year during his frequent road trips for his business, which maintains computer networks for small companies. But it was not just a need to save money on gas that prompted him to buy an alternative-fuel vehicle.

"I was motivated by the technology, and it's so good for the environment," he said.

A growing number of consumers appear to share his interest. According to CNW Marketing/Research of Bandon, Ore., which studies consumer behavior, about 37 percent of car shoppers in 2001 considered themselves "extremely concerned" about the environment, more than triple the 11 percent who put themselves in that category in 1996.

Still, sales of alternative-fuel vehicles make up only a fraction of the overall market for cars and trucks — about 50,000 of more than 16 million vehicles sold nationwide this year. That is partly because they are not as plentiful in the United States as they are elsewhere, where gas prices tend to be much higher. And drivers who want to switch will probably have to make some adjustments: the new fuel-efficient vehicles lack some of the convenience and, in some cases, the comforts of conventional ones. They may also be more expensive than gasoline-powered cars of similar size.

Diesel power, the most familiar alternative for improving fuel economy, is a popular choice for buyers of big pickup trucks and sport utility vehicles like the Ford F- Series, the Dodge Ram and the Chevrolet Suburban. Analysts expect sales of about 500,000 diesel-powered trucks and S.U.V.'s this year, about the same as last year.

But for Americans, finding a diesel vehicle can be difficult. The chief source now is Volkswagen (news/quote); diesel versions of its New Beetle, Jetta and Golf have been flying out of showrooms this year.

Dealers exhausted their supply of 2001 models by mid-July and since then have been taking orders for the 2002 versions, which should start arriving this month. If sales continue at their year-to-date pace, VW dealers will sell about 23,000 diesels this year, about 8 percent of VW's total sales of 365,000 vehicles in the United States. While dealers are urging Volkswagen to ship more diesels to America for the 2002 model year, which starts this fall, VW's supply is limited because its diesels are so popular abroad. In Europe, more than half the cars sold are diesel-powered; the figure is as high as 75 percent in Italy and France.

All diesel cars sold by VW in the United States have a 1.9-liter, 90-horsepower turbo- charged engine, which gets above-average gas mileage — 42 m.p.g. in city driving, 49 on the highway. (By contrast, a gasoline Golf, with a 2.0-liter engine, gets 22 m.p.g. in the city and 28 on the highway.) Unlike past diesels, which spit and chugged and were generally noisy, the modern VW diesels with redesigned engines are smooth and quiet, providing relatively peppy acceleration, given the engine's small size. (Prices start at around $17,600 for the Golf, $19,300 for the New Beetle and $19,700 for the Jetta — slightly higher than the nondiesel models.)

The primary drawback is the same frustration known by many past owners: finding fuel. Gas stations in some cities and suburbs do not carry diesel, so the primary source is truck stops, which can be inconvenient.

Availability of fuel is not a problem for owners of hybrid gas-electric cars like the Insight and the Prius (pronounced PREE- us), which use unleaded gasoline to bolster the thrust of an electric motor. Those cars also have an advantage over recent, purely electric cars like the EV1 from General Motors (news/quote), which proved too expensive and quirky to find wide acceptance. The EV1, introduced in 1996, was available only in California and Arizona and was discontinued in 1999.

Unlike electric cars, hybrid gas-electric cars need no special equipment, like battery-charging stations.

"A lot of people are surprised that you don't have to plug them in," said Ernest Bastien, corporate marketing manager at Toyota Motor Sales USA, who is in charge of American sales for the Prius. The car became available in Japan in 1997 and in the United States last year.

But first, you have to find one. Both the Prius and the Insight are in short supply — the Prius is sold out until April, while the Insight can be extremely scarce in markets like California, where they are most popular. (The City of New York just bought 231 of them, while New York State and New Jersey bought several dozen to be used by municipal and state agencies.)

Of the pair, Prius is clearly the more conventional in appearance. It closely resembles the Toyota Corolla subcompact, on which it is based. It has an automatic transmission, four doors, seating for five people, plenty of interior room and a large trunk. But it costs about $20,000, or at least $5,000 more than a comparably equipped Corolla. Toyota expects to sell about 14,000 Prius cars during this calendar year.

The Insight, which also starts at around $20,000, makes a bolder statement. It is a low-slung, two-seat hatchback with an aluminum body that brings to mind an inverted bathtub. It has a 1.0-liter, three-cylinder engine and until the 2002 model year was available only with a manual transmission. Honda now offers an automatic.

If the Insight lags behind the Prius in practicality, it whooshes ahead in fuel economy, getting 61 m.p.g. in city driving and 68 on the highway. Many owners say they have achieved 70 m.p.g. or more under optimum conditions.

While the Prius is not as frugal, it performs better in city driving than on the open road. It is rated 52 m.p.g. city, 45 m.p.g. highway. Both cars are more efficient at slower speeds, which make greater use of electric power, than at high speeds, which require a blast of gasoline.

To achieve those figures, however, Honda and Toyota encourage owners to drive sedately, without quick starts or stops. "If you're a hot-rodder, and you expect to get the numbers we claim, you're going to be disappointed," said Andrew Boyd, a spokesman for Honda in Detroit.

Jeff Harris, a Chicago lawyer, says he is averaging 55 m.p.g. in his Insight because he spends so much time in stop-and-go traffic. Mr. Harris bought his Insight last April, before gas prices climbed in his area, and bargained his dealer down to $15,200, so he considers his car a good value.

Mr. Harris, 39, says there is just one irritation about owning an Insight: the attention it draws.

"Sometimes it's an annoyance, because people want to stop and talk to you all the time," he said of his car.

Soon after he bought it, Mr. Harris recalled rolling up to a four-way stop, along with three other cars. He waited for one of the others to proceed. But none did: they were all staring at his car.

The Insight and the Prius are about to have challengers in the hybrid market. Following Toyota with something more conventional, Honda will introduce a hybrid version of its subcompact Civic, due early next year. Chrysler plans a hybrid Dodge Durango sport utility, in spring 2002, while Ford plans a hybrid version of its small Escape S.U.V.

Mr. Alden, the Prius owner, says he is delighted with his new car and is looking forward to having more hybrid vehicles on the market.

"It wasn't long ago that American manufacturers were saying, `this can't be done,' " he said. "Well, this really is what the public is buying."