April 25, 1981

The end of the 15-month curb on grain exports to the Soviet Union was probably President Reagan's most important single foreign policy decision so far, but there was no indication today from people in the Administration that it would do anything but undercut Washington's proclaimed policy of toughness toward Moscow. Hours after the decision was disclosed, Secretary of State Alexander M. Haig Jr. inveighed, as he has done repeatedly since taking office, about the Soviet Union's posing ''the greatest danger'' to the world. He promised that the United States would never accept Soviet intervention in countries such as Afghanistan. But his aides acknowledged that Mr. Haig's speech seemed to have a hollow ring because of the decision to end the grain curbs, which were imposed by the Carter Administration in retaliation for the Soviet move into Afghanistan in December 1979.

For nearly two years, two tribes, one established and one new to this fishing village on the grayish, sunflecked waters of Galveston Bay, have been locked in a kind of territorial struggle as old as the species. Now the struggle is approaching a critical point. One tribe is made up of American shrimp fishermen. The other consists of Vietnamese refugees who, having fled their country after the fall of Saigon in 1975, flocked to the Gulf Coast to make their living as shrimpers, too. Operating from a stretch of docks that has been dubbed ''Saigon Harbor,'' they now outnumber the Americans. The problem, according to the white shrimpers, is that their livelihood is jeopardized because the shrimp fishing hereabouts will simply not support everybody. In the resulting dispute, two Vietnamese shrimp boats have been burned, and hostility and suspicion are rife in the twin fishing towns of Seabrook and Kemah.

Under a barrage of hostile questions from the prosecutor, Senator Harrison A. Williams Jr. maintained yesterday that he had neither said nor implied to undercover agents that he would arrange to get Government contracts for a titanium mine in which he had accepted stock. He continued to insist this was so even after the prosecutor, Thomas P. Puccio, played a portion of a 1979 videotape in which the New Jersey Democrat told the agents there would be ''no problem'' in using his relationships with top Government officials in trying to get the contracts. The tape was made secretly as the Senator was meeting in a Virginia motel with the agents, one of whom posed as an Arab sheik. It is a key piece of prosecution evidence in the 61-year-old Senator's Abscam bribery trial, which ended its fourth week yesterday in Federal District Court in Brooklyn. The cross-examination began in late afternoon.

A Federal judge ruled yesterday that a New York City pension system, which assumes that women live three years longer than men, illegally discriminates against women employees by requiring them to make higher contributions even though they receive lower monthly benefits when they retire. The judge, Mary Johnson Lowe of District Court in Manhattan, ruled in a 31-page opinion that the higher contributions and lower benefits violated the Civil Rights Act of of 1964. Judge Lowe said she would not decide what damages should be awarded before further hearings or a full trial. Her decision yesterday was based on legal motions only.

The number of measles cases reported in the United States so far this year is 80 percent below the comparable period last year, a drop so dramatic that the Federal Centers for Disease Control predicts that, except for imported cases, the disease will be eliminated by the end of 1982. The Atlanta-based agency said yesterday that even though the measles season was well under way, there had been only 778 known cases in the first 14 weeks of this year, as against 3,897 cases in the comparable period of 1980. The season ordinarily peaks in April. Dr. Alan Bloch, a medical epidemiologist for the disease control agency, said the precipitous drop was an indication of the success of nationwide vaccinations under the Measles Elimination Program. The program, using a vaccine first licensed in this country in 1963, was initiated two and a half years ago by the Department of Health, Education and Welfare, now the Department of Health and Human Services.