(FORTUNE Magazine) – Remember the dinosaurs? Well, a similar fate may await Explorers, Blazers, Pathfinders, and the other hulking sport-utility vehicles (SUVs) that have dominated the automotive landscape in recent years. Demographic trends and changing tastes are part of the reason the explosive growth of the SUVs will slow, but the real killer is the arrival of a new species of vehicle better adapted to today's market.

The first of these New Age off-roaders to arrive in the U.S. is the Toyota RAV4. With brawny bumpers, plastic body-side cladding, and rear-mounted spare tire, it looks like the athletic younger sister of a Land Cruiser. But its light weight, nimble handling, and peppy performance make it far easier, cheaper, and more entertaining to drive. It's got all the flavor of the old sport utilities, but it's less filling. Think of it as SUV Lite. In May, Toyota sold nearly 6,000 RAV4s at prices ranging from $15,368 to just over $23,000. Owners are enthusiastic, as is the trade press. After a thorough examination, Autoweek pronounced: "In its own way, the RAV4 is just as big a deal today as Chrysler's first minivan was 13 years ago."

Maybe bigger. If Lites indeed ride roughshod over SUV sales, Detroit is going to feel real bottom-line pain. The Big Three make all their auto profits from light trucks these days--SUVs, plus minivans and pickups. Margins are fat because there is relatively little competition. Most Japanese SUVs are imported from Japan, which puts them at a price disadvantage.

Lites are superior to conventional SUVs because of the way they are built. Almost all current sport utilities are the offspring of rugged pickup trucks. Their bodies rest on thick steel rails, and they ride on suspensions not so different from the leaf springs used on stage coaches and farm wagons. Lites, descended from passenger cars, have lightweight unibody frames and modern suspensions. Drivers sit up high--with four-wheel drive at their command, looking like they have just crossed the Sahara--but they also enjoy passenger-car handling and ride free of the annoying bucking and rocking that you get in the old SUVs. Toyota's RAV4 (the name signifies Recreational Active Vehicle with four-wheel drive) shares its basic mechanical underpinnings with the sporty Celica, and it's quieter, more fuel efficient, and less expensive than its big, old competitors.

Toyota's SUV Lite monopoly in the U.S. market lasts only until the end of the year, when Honda introduces the CR-V. Based on the Civic passenger car and slightly larger than the RAV4, the CR-V will start at prices just under $20,000. In Japan, where both vehicles are already on sale, the CR-V is outselling the RAV4 two to one. Just wait until it hits the U.S., says George Peterson, a California-based automotive consultant: "It's bigger, looks more purposeful, and is a better value."

New competition isn't the only threat to traditional SUVs. Demographics are working against them too. As car buyers age, they are less willing to put up with the rough ride, clumsy handling, and all-around crudeness. They want to be pampered with pillow ride and plush surroundings. The percentage of buyers considering buying an SUV drops off steeply at age 50, which just happens to be the leading edge of the baby-boom generation.

The fickleness of the zeitgeist may pose an even bigger danger for SUVs--old and new alike. Consumer research shows that many SUV owners (87.5%, according to one marketing report) buy them because they think the vehicles make them look cool. And image, of course, is a transitory thing. Just as padded vinyl roofs and fender skirts lost their cachet and sales appeal, so may SUVs. As Paul Ballew, chief economist for J.D. Power & Associates, puts it diplomatically: "A note of caution has to be expressed over the more upbeat assessments coming from the industry. We see the market leveling off in the next decade."

So have U.S. automakers allowed the Japanese to steal a march on them again? Yup. Says one Detroit insider: "It is tough to look at new ideas when you already own the SUV market and you're selling every one you can make. Besides, there is also the danger of cannibalizing your existing products." Blame institutional inertia too. Since GM, Ford, and Chrysler all have separate engineering staffs for cars and trucks, there has been little incentive for the truck guys to spread the SUV action around.

For the time being, the Big Three have little to worry about. Ford sells far more Explorers every week than Toyota does RAV4s in a month. Until the Japanese Lites can be produced in the U.S. instead of Japan, volume will be limited. And although most people never drive farther off-road than valet parking--and thus hardly need all that power and heft--there is no substitute for a V-8 powered Suburban or Grand Cherokee if you're navigating the Continental Divide or towing an Airstream trailer.

Consultant Peterson says 11 more subcompact and compact Lites are on their way to market, and he forecasts these car-based vehicles could capture 20% of the SUV business in 2005, vs. nearly zero today. Ford is believed to be thinking of a Contour-based SUV, while Chrysler is said to be looking at the Neon as a starting point. But both projects are several years from execution. For now, the best Lite competition that Detroit can offer is pretty feeble: a Taurus wagon or a four-wheel-drive Chrysler minivan. The question isn't whether the Big Three will lose market share. It's how much.