THEY'VE DONE IT again. Early last week, U.S. government officials, citing the 1952 McCarran Act, refused to allow a visitor into the United States on political grounds. Usually these decisions are made by the State Department, which has the power to deny visas to persons whose presence here is deemed "prejudicial to the public interest" or dangerous to the "welfare, safety or security of the United States." This time -- because Canadians don't need visas -- it was the Immigration and Naturalization Service that stopped wildlife writer Farley Mowat at the Toronto airport. By Friday, the prohibition was lifted for Mr. Mowat, but the statute and the policies implementing it remain unchanged.

The McCarran Act was a piece of xenophobic legislation enacted in the early 1950s whose guiding emotion was fear -- fear that the wrong people would get into this country and overwhelm or subvert us. Its provisions are regularly invoked to keep people out of the country who might say something the government is afraid to have us hear. The act gave broad powers to the bureaucracy to exclude would-be visitors. The statute reflects a profound misunderstanding of American free-speech traditions and sadly underestimates the critical judgment of a free people.

Farley Mowat is a Canadian who writes about the wilderness. His book "Never Cry Wolf" is a classic study of these predators and was made into a popular movie. His latest work is about wildlife on the seacoasts of the United States and Canada. He may have made comments about American military power and may have joined a committee in support of Castro's Cuban government many years ago. So what? Surely he does not belong on any list, book or computer file of persons who pose a danger to the country.

How do these lists get compiled anyway? The State and Justice departments won't say, but once you're listed, you're there to stay unless the attorney general gives you a special entry waiver every time you want to come into the United States. The fact that Mr. Mowat was granted a waiver does not solve the general problem. An unknown number of others are on those lists, some because of affiliations or activities that took place decades ago.

The law is preposterous and outmoded. A country as strong, stable and free as ours can hear Mr. Mowat, Nicaraguan Cabinet members (cultural affairs minister Ernesto Cardenal is still waiting for a required waiver), Chinese party leaders, the Rev. Ian Paisley and even Mikhail Gorbachev without falling under their spell. Rep. Barney Frank is planning to introduce legislation to change this law, and his effort deserves support. These offensive restrictions should be repealed.