DETROIT, Nov. 1 - October, which is the start of the new model year, used to be a month for the auto industry to celebrate. This year, it was a month for Detroit to forget.
General Motors, Ford and Chrysler held their lowest shares of the American market ever last month. Sales fell in the wake of high gasoline prices, fears about the economy and consumer resistance to buying cars without the big discounts the companies offered this summer.
General Motors and Ford were hurt the most, with G.M. sales dropping 25.6 percent compared with October 2004 and Ford sales down 26.1 percent.
Big sport utility vehicles, on which Detroit based much of its resurgence in the 1990's, did even worse, with sales of some of the largest S.U.V.'s, the Ford Expedition and the Cadillac Escalade XLT among them, falling by half or more.
But Asian auto companies performed much better, recording their biggest share of the American market ever.
According to Autodata, which tracks industry sales, Toyota, which ranked third in sales last month, came within just one market share point of the No. 2 Ford. Toyota's overall sales rose 1.3 percent in October.
Car sales, though down at G.M. and Ford, increased at Chrysler, which has enjoyed success with models like the big 300C and which had strong sales to fleet customers like rental car companies. Amid that strength, Chrysler's overall decline in October sales was only 3.1 percent.
The Detroit automakers have given no sign that they will restore the employee discount plans that brought masses of consumers into showrooms in June, July and August, leading to record monthly sales.
Instead, they are likely to wait until their year-end closeout sales to offer more generous deals.
"This is a period when we've just got to take the medicine and swallow it," George Pipas, a sales analyst at Ford, said Tuesday.
But anything less than this summer's deals - which doubled the rebates on some models offered by Detroit - will probably not work as well with prospective buyers, who have been rattled by higher gasoline prices, analysts said.
Even though gas prices have fallen from their highs after Hurricane Katrina struck the Gulf Coast, "people are a lot more cautious because they know it can go to $3 a gallon," said Jesse Toprak, the executive director of industry analysis at Edmunds.com, a Web site that offers consumers advice on buying cars.
"A few years ago, that was only a rumor," he said.
Amid all the economic unease, industry sales fell 10.9 percent last month, according to an estimate by Ward's Automotive Reports, a statistics company.
Together, G.M., Ford and Chrysler held just 52.2 percent of the American market in October, the smallest ever, according to Autodata, which also tracks the industry (55.3 percent, including foreign subsidiaries, Ward's AutoInfoBank reported).
During the summer sales boom, they had nearly 10 more percentage points of market share. The Asian automakers, meanwhile, held 40 percent of car and truck sales in the United States last month, with Toyota, Honda and Hyundai all reporting increases.
The gloom in Detroit on Tuesday was in sharp contrast to the summer's euphoria, when buyers' record demand helped clear away stocks of 2005 model cars much earlier than usual.
But just as swiftly as sales spiked, they crashed once the so-called employee discounts disappeared.
"It's like the bigger they are, the harder they fall," Mr. Pipas at Ford said. And the bigger the vehicle, it seemed, the worse its sales last month.
The sales of big sport utility vehicles all dropped by more than 50 percent. Foreign companies were not immune: sales of the Toyota Land Cruiser and the Nissan Armada also dropped by half or more.
Not long ago, those vehicles generated thousands of dollars each in profit, with some customers waiting months for their vehicles to arrive.
The decline of the big S.U.V.'s comes as G.M. is preparing to roll out a new line of pickup trucks, code-named the GMT-900's, on which it is depending for a much-needed sales boost.
G.M., whose market share four decades ago was as large as all three Detroit companies put together now, fell to 22.2 percent of the American market last month, according to Autodata.
Paul Ballew, G.M.'s chief sales analyst, termed October "lackluster," but said he was not overly concerned about the drop in sales given the "overwhelming amount of negative news and press coverage" in the last few weeks.
"We don't want to read too much into volatility and short-term shocks," he said, adding, "Let's see how things play out in November and December."
But as the October results showed, consumers are more inclined to buy Detroit vehicles when they are offered substantial, easy-to-understand incentive plans, Mr. Pipas said.
At least for now, Ford does not intend to roll out another such program, even though its market share last month fell to 16.1 percent of American-brand car and truck sales.
Instead, it will concentrate on more modest incentives aimed at slow-selling vehicles, which, Mr. Pipas conceded, would be "like pushing on a string."
Chrysler, though, waded back into the incentive race on Tuesday, saying it would offer rebates of at least $1,000 on every 2005 and 2006 vehicle. And Toyota said it would expand its rebates on S.U.V.'s, where October sales fell 16 percent.
Gary G. Dilts, the senior vice president for sales, said: "We know we've got consumers out there that are concerned about a number of issues, whether it's fuel prices or heating or automotive or general economic concerns. We feel the need to put some kind of incentive out there that answers the question, Why buy now?"
What some consumers are in fact buying now are cars. Ford reported stronger-than-expected sales for three new models, including the midsize Ford Fusion, a successor to the Taurus sedan, which received favorable reviews on its introduction. Mr. Pipas called the Fusion "an oasis in the desert" for Ford, where trucks still outsell cars by 2 to 1.
Honda, meanwhile, said a new version of its small Civic had gotten a warm response, while sales of its car-based pickup truck, the Ridgeline, were strong last month, bucking the trend to slower truck sales.