Sales of light vehicles in Mexico fell 1.8 percent, to 68,158.
Despite the decline, the Mexican market through September remained 10.0 percent ahead of a record sales year in 2000.
September's drop was the first of the year in Mexico. And it was the first time a monthly sales total had dropped compared with the year-ago month since June 1999, when sales dipped 1.5 percent.
Mexico's plunge echoed a 12.7 percent drop in Canadian sales in September and a 12.5 percent decline in the U.S. market during the month of the terrorist attacks.
General Motors de Mexico was down 21.1 percent, giving it a 0.4 percent decline for 2001.
DaimlerChrysler sales rose 18.0 percent; for the year, its Mexican sales are up 27.2 percent.
Ford rose 6.9 percent, in comparison with a 12.5 percent increase for the year.
Nissan and Volkswagen posted declines last month but have posted gains for the year.
Mexican production continued to fall. September output was off 7.8 percent, primarily because of double-digit drops at Ford and Volks-wagen.
So far, production in Mexican factories has fallen 2.4 percent.
If that trend continues, this year would mark the first year-to-year drop in Mexican production since 1995.