MEXICO CITY (AP) _ A Mexican truck manufacturer completed the purchase of Arizona-based bus maker Motor Coach Industries International on Monday in a $300 million deal said to be the first such acquisition under the North American Free Trade Agreement.

Phoenix-based Motor Coach will become a wholly owned subsidiary of Mexico's Grupo Dina consortium, the companies announced. The deal, first announced in November, had to be renegotiated after a drop in the price of Dina stock.

''The newly combined company will be well-positioned as a global competitor with strong market positions in Mexico, the United States and Canada,'' said George Aucott, Motor Coach chairman and chief executive officer.

Dina is a leading supplier of heavy trucks and tractor-trailers in Mexico.

Motor Coach is the biggest manufacturer of long-haul buses in North America with factories in Winnipeg, Manitoba, and Pembina, N.D. About 2,300 people are employed at those plants and at the headquarters.

The bus bodies are welded in Winnipeg and finished in Pembina in an operation that has a capacity of 1,300 buses per year. About 897 buses were produced in 1993 and the company expects to make 975 this year, said Craig Lentzsch, executive vice president of Motor Coach in Phoenix.

The deal doesn't change management's plans to sell or close a division that manufactures city buses, Lentzsch said. The city bus division employs 900 people at a manufacturing plant in Roswell, N.M., and about 100 at a bus- refurbishing plant in Schenectady, N.Y.

Motor Coach was a division of Dial Corp until it was spun off into a separate, publicly traded company last year.

''We are confident that the benefits of this transaction will become apparent through our broader product lines and greater penetration of the international markets,'' said Ernesto Moya, Dina's chief executive officer.

The deal links businesses in all three countries involved in NAFTA. Tariffs on most goods will be gradually shed over 15 years as a result of the treaty.

''The merger represents the first time since NAFTA's establishment that a Mexican company has acquired 100 percent of the outstanding shares of a publicly held U.S. company,'' said Luis Vinicio Anduaga, a spokesman for Grupo Dina.

The deal involves a swap of former Motor Coach common shares for the right to receive Dina American Depositary Shares or a portion of Dina convertible subordinated debentures.

At current prices the swap is worth about $300 million, company officials said.