The federal government has prevented a prominent South African social scientist from speaking at the annual meeting of the American Sociological Association by failing to respond to his visa application. The move has drawn protests that the government misused its visa authority to keep him out of the United States because it does not like his political views.

The scholar, Adam Habib, is an American-trained Ph.D. and executive director of the South Africa-based Human Science Research Council’s Program on Democracy and Governance. Mr. Habib is also a Muslim of Indian descent who has been a vocal critic of the war in Iraq and certain terrorism-related policies of the U.S. government. He had been scheduled to speak at the sociology meeting, which is under way in New York City.

Last October Mr. Habib was detained when he arrived at a New York airport for a series of academic meetings in the United States. His visa was revoked without explanation, and he was put on a flight back to South Africa. His experience mirrored those of several other foreign academics in recent years who have been barred from the United States, with no reason given.

In a written statement the sociology association said the government’s failure to issue Mr. Habib a visa looked like a case of “ideological exclusion.” If so, the statement said, “this raises significant concerns for the association’s leaders and its more than 14,000 members about the fate of academic freedom.” The American Civil Liberties Union also protested the visa denial. —Burton Bollag