THERE'S A FIGHT at the Food and Agriculture Organization of the United Nations. It is about the way the agency should do its work of increasing agricultural production in the Third World. It is also about power: not merely what person should run the agency but the perennial U.N. issue of whether the many small members or the few large donors should be in control.

The occasion is the FAO election coming up Nov. 9. Director General Edouard Saouma, a Lebanese whom his numerous critics find assertive verging on unbearable, is running for an unprecedented third six-year term. The challenger is Moise Mensah of Benin, No. 2 at the International Fund for Agricultural Development. Africa's nominee, Mr. Mensah is supported by countries, including the United States, that provide nearly three-quarters of the FAO budget.

In the old days, the colonial powers ran the colonies' agriculture their way, or ignored it. After World War II, FAO became a Western-driven vehicle of Third World development, but in the Third World's political surge of the 1970s, Dr. Saouma took the wheel. A strong manager whose most bitter foes distinguish his style from that of UNESCO's corrupt and now-departing Amadou Mahtar Mbow, Dr. Saouma has been all business, permitting no anti-Israel nonsense. But he has done things very much his way. This has meant going beyond the technical assistance and policy coordination favored by the big donors into loan programs spread out thinly among the agency's 156 members. It has meant using the director general's powers to dispense jobs and contracts to enforce his will.

Enter Canada. Canada makes involvement in the politics of world agriculture a focus of its foreign policy, and it has led an opposition consisting of most industrialized democracies. Their broad concerns include their status in the system and, beyond that, the effectiveness of FAO's programs. Their immense frustration with the hardball ways of Mr. Saouma is palpable, though few other donors are as ready as the Canadians to let it all hang out.

The United States favors the challenger, seeing in Mr. Mensah someone likely to be more respectful of the membership and more sympathetic to free-market principles. The American government is, in this instance, pleased to let Canada get out front. For one reason, it has paid only a third of its 1986-87 dues, and being in arrears, it finds the American voice does not carry. Tactically, discretion may be appropriate. But the real fight -- the fight against world hunger -- requires a vigorous American role.