HOUSTON (AP) _ The number of working oil and gas rigs in the United States fell by 28 to total 880 this week, Baker Hughes Inc. announced.

The rig count, the widely watched industry index of drilling activity, gained ground the week before, rising by four to total 908.

A year ago, the count totaled 780, Baker Hughes announced Monday.

Baker Hughes' rig count reflects the number of rigs actively exploring for oil as of last Friday, not the number of rigs producing oil.

Houston-based Hughes Tool Co., an oil toolmaker company that merged with Baker International Corp. of Orange, Calif., has kept track of the number of working rigs since 1940.

The rig count reached a peak of 4,500 active rigs in December 1981, which was the height of the oil boom. But oil prices plunged to less than $10 per 42-gallon barrel in the summer of 1986 and the rig count went with it, dropping to 663 - the lowest since Hughes began compiling figures.

Among major oil producing states, Texas saw its rig count rise by four, while Pennsylvania's rig count rose by three, and New Mexico and Wyoming's rig counts each rose by one.

The biggest weekly loser was Louisiana, where the rig count dropped by nine, followed by Colorado, with a loss of seven. Michigan and Oklahoma's rig counts each dropped by five, while the rig counts in Ohio and Kansas each dropped by three, and North Dakota lost one rig.

California's rig count was unchanged from the week before.