American beer manufacturers, angered that they can't sell their suds south of the border, took aim yesterday at Mexico's preferential treatment as a Third World nation that allows its beer into the United States duty-free.

They asked that Mexican beer, which amounts to 6.2 percent of all U.S. beer imports, be graduated from the Generalized System of Preferences list and charged duty.

The U.S. Brewers Association petition appears to be part of a campaign by American beer manufacturers -- especially No. 1 Anheuser-Busch -- to get Mexico to allow U.S. beer to be sold in Mexico.

"We believe in free competition as long as it is free on both sides of the border. It is important that American brewers receive fair treatment by the Mexican government," said Anheuser-Busch President Dennis P. Long in a July 2 memo to his company's wholesalers that listed action on Mexican beer's duty-free status as the first step in a campaign to open markets for U.S. beer.

The memo, which was included as part of Mexico's counterattack on the Brewers Association petition, urged wholesalers of Anheuser-Busch who also distribute Mexican beer to "let your Mexican suppliers know that it is time for their free ride to end."

Bart S. Fisher, attorney for Mexico's National Association of Beer Manufacturers, accused Long before the International Trade Commission of "predatory corporate behavior."

"For what does the mighty U.S. beer industry have to fear from Mexican beer?" Fisher continued.

He said the $26.6 million of Mexican beer imported by America last year amounted to 6.7 percent of all U.S. beer imports. "America is hardly awash in Mexican beer, nor is it likely to be," Fisher told a hearing yesterday of the U.S. Trade Representative's Trade Policy Staff Committee.

But the Brewers Association argued in its petition that Mexico is the world's sixth largest producer of beer, and its impact on the American Southwest is growing rapidly. In California, Mexican beer was 26 percent of the import market in 1981.

A Brewers Association representative spokesman said the group supports the statement of Matthew J. Marks, attorney for Anheuser-Busch, who focused on the Mexican government's refusal to allow U.S. beer to be sold in Mexico.