Sales of medium- and heavy-duty trucks fell 7.4 percent in September from the same month a year ago, to 25,716 units.
Sales of Class 4-8 trucks have been falling since February 2007, but the pace of decline eased in September. In August, sales plummeted 22.6 percent.
In the first nine months of the year, Class 4-8 sales fell 19.7 percent from the year-earlier period, to 248,009 trucks.
Sales of large trucks were mixed last month.
Sales of Class 8 trucks climbed 12.5 percent to 10,891 units, but that was the only class to post a gain.
Class 7 trucks took the biggest hit, plunging 26.5 percent to 3,535 units. Sales of Class 5 trucks slid 22.9 percent to 2,403 units.
Class 6 sales tumbled 24.7 percent to 2,869 units.
Sales of Class 4 trucks fell 5.2 percent to 6,018 units.
International sold 5,689 units, up 21.9 percent, to take first place among truckmakers last month. International inched out Ford, which sold 5,659 units, down 23.8 percent. Freightliner's sales rose 8.9 percent to 4,830 units for third place.