In September, industrywide sales of large S.U.V.'s were down 43 percent from a year earlier, according to Ward's AutoInfoBank. That is particularly bad news for General Motors and the Ford Motor Company, which are dependent on truck-based S.U.V.'s.
Last month, G.M.'s overall sales fell 24.2 percent and Ford's declined 20.3 percent, compared with the same month a year earlier.
In contrast, Japanese carmakers reported increases last month, propelled by passenger cars and smaller S.U.V.'s known as crossover vehicles. Toyota's sales rose 10.3 percent, Honda's increased 11.7 percent and Nissan's, 16.4 percent.
Overall auto sales fell 7.9 percent last month, to a seasonally adjusted sales rate equivalent to 16.3 million vehicles if it were maintained for an entire year.
Part of the reason sales cooled is that the domestic automakers are phasing out their employee discount offers, which were mostly used to clear out 2005 models. The companies have also offered the deals on some 2006 model S.U.V.'s and pickup trucks, but have said the discounts will not continue this month, though company officials have been known to change their minds.
With President Bush joining calls for conservation at the pump, automakers are finding that this is not an ideal time to sell their least fuel-efficient models. Gasoline consumption has fallen for four consecutive weeks as Americans cut back, and a wide range of consumer surveys show that at least some Americans are looking to conserve and are changing their minds about what kind of car or truck to buy.
After G.M. and Ford announced their September sales results, Standard & Poor's, citing high gasoline prices, put both companies on negative credit watches for possible downgrades deeper into junk bond territory. S.& P. also noted that G.M. planned to release a new generation of its medium and large S.U.V.'s early next year into what many analysts fear will be an inhospitable market.
"We believe soaring gasoline prices after Hurricanes Katrina and Rita are leading to an accelerating decline in demand for S.U.V.'s," said Scott Sprinzen, a credit analyst at Standard & Poor's, adding that the reception given to G.M.'s new offerings would be critical, given the company's "disproportionate reliance on S.U.V.-related earnings."
For the moment, though, big S.U.V.'s are bearing the brunt of consumer discontent. While sales have been diminishing all year long, September was particularly ugly.
At Ford, sales of the Explorer, Expedition and Lincoln Navigator fell more than 50 percent compared with the same month a year earlier. The company also built its last Excursion in the month, ending production of its largest S.U.V.
At G.M., sales of the Chevrolet Suburban and Tahoe fell more than 50 percent, while the GMC Yukon was down 46 percent and the Cadillac Escalade fell nearly 23 percent, with the supersize Escalade ESV falling 40 percent. Sales of the Hummer H2, made by G.M., fell 32 percent.
The sharp declines come despite heavy spending on discounts. According to a new estimate from the auto tracking firm Edmunds, automakers are spending twice as much on discounts for each large S.U.V. they sell - $4,704 - than the $2,366 overall average vehicle incentive. That is partly mitigated by the fact that the vehicles tend to be more expensive, but only partly.
"Just three years ago, people ignored the fuel economy numbers on the sticker," said Jesse Toprak, an analyst at Edmunds. "Now it's one of the first things people ask about, especially the middle-income families."
Those families have been the core market for large S.U.V.'s.
George Pipas, chief industry sales analyst at Ford, said "sales declines, this year at least, in the traditional S.U.V. segment are becoming the norm, and we expect that to continue to be the case in the near future."
Large models at Japanese automakers also suffered, with the Toyota Land Cruiser off more than 50 percent and the Nissan Armada down 21 percent. But the Japanese make more of the smaller and lighter sport utility vehicles that customers are shifting to, and have a stronger hold on the resurgent car market.
Consumers are also snapping up as many hybrid electric vehicles as the industry produces, with sales of the Toyota Prius up 90 percent and the hybrid version of the Ford Escape having a record sales month.
Perhaps Detroit's only bright spot was that sales at the Chrysler division of the German automaker DaimlerChrysler held up, rising 4 percent, driven by a 26 percent gain in its car business.
According to the Autodata Corporation, sales of the traditional domestic automotive brands fell to 55 percent of the overall market, from 61.6 percent in September 2004.
G.M. has said the large S.U.V. market will perk up once it introduces its new generation of such vehicles at the beginning of next year.
But at a media briefing last week, Robert A. Lutz, G.M.'s vice chairman and product development chief, emphasized that his company would be hedging its bets. "I'm betting we're going to see regular under $2 a gallon again," he said, but added that "what we have is volatility and the volatility may continue.
"That's why we're not depending solely on the new full-size sport utilities," Mr. Lutz said. "We're going to be doing a balanced portfolio of high-fuel-economy passenger cars, high-fuel-economy crossover vehicles."
Consumer surveys over the past month have shown that people are more aware of prices at the pump.
Twenty-eight percent of S.U.V. owners surveyed last month by AutoPacific Research, a California consulting firm, said they would shift to a different kind of vehicle for their next purchase. A quarter of drivers whose vehicles had the larger V-8 engines said they would look for a less powerful engine next time. The firm talked to 1,313 people for the survey.
A Kelley Blue Book and Harris Interactive Survey of about 2,000 adults in late August found that gas prices had either changed people's minds "or strongly influenced purchase decisions."
In a September survey sponsored by AARP of 568 people over 50 years old, 47 percent said they were reducing travel and vacations and 39 percent said they were visiting family and friends less often. Thirteen percent of the respondents even said they were offsetting high gasoline prices by eating less.
"These are lifestyle-changing prices," said James Toedtman, editor of the AARP Bulletin, in a statement. "Many people were already making significant adjustments before Katrina. Poststorm costs have driven Americans to find additional ways to trim their household budgets."