A number of beaches have been closed to swimming in Oak Bluffs and West Tisbury due to bacterial contamination.

Inkwell Beach, Pay Beach, Eastville Beach and Madeiras Cove in Oak Bluffs have all been closed to swimmers, Oak Bluffs health agent Shirley Fauteux confirmed on Wednesday. Ms. Fauteux said elevated bacterial counts were found in water samples taken on Monday.

In West Tisbury Sepiessa Point is closed, and at Long Point reservation, Great Cove, Long Cove and the ocean are all closed to swimming. West Tisbury health agent John Powers said Wednesday that he had resampled on Tuesday night and was waiting for results.

Mr. Powers said the bacteria is entrococci, an indicator organism. The cause of the contamination remains unclear.

“It’s very unusual. I wish I knew [what caused it],” Mr. Powers said.

The beaches will remain closed until at least tomorrow as results from samples taken from area beaches today become available.

All Edgartown and Tisbury beaches remain open.