Negotiators for Mack Trucks Inc. and the United Auto Workers reached agreement on a tentative contract yesterday, the ninth day of a strike by 9,200 workers in three states, officials said.

Employes are to return to work this morning, and the production plants should resume normal operations Wednesday, said Don Miller, a spokesman at the Allentown, Pa., headquarters of Mack, the nation's second-largest manufacturer of heavy-duty trucks.

As a condition of the agreement, the union agreed to suspend the strike, which had shut down production at plants in Allentown and Macungie, Pa., and Hagerstown, Md. A distribution center in Bridgewater, N.J., also was closed by the walkout, the first against Mack in two decades.

The three-year agreement was announced by William E. Walker, Mack senior vice president for human resources, and UAW Vice President Bill Casstevens.

A vote on the agreement will be held Nov. 11, said James Alles, president of UAW Local 229 in Bridgewater. Details won't be released until the ratification, both union and Mack officials said.

Nearly 450 workers at Mack's assembly plant in Oakville, Ontario, were laid off because of the strike, which slowed the flow of parts to the Canadian facility. The Canadian employes probably will not return to work today because of lag time in parts reaching the Canadian plant once the U.S. production resumes, Miller said.