Inkwell Beach in Oak Bluffs has been closed to swimming after water tests showed unacceptable levels of bacteria.

Signs have been posted to warn bathers of the bacteria.

David Caron, chairman of the Oak Bluffs board of health, reported that water samples taken Monday had come back with an enterococci count of 159 parts per milliliter; the maximum acceptable is 100.

There was not a clear reason for the unusually high reading, Mr. Caron said. It comes after a series of inexplicable readings and beach closures. Inkwell and several other beaches were closed temporarily just two weeks ago, in the height of the summer tourist season as well.

New samples have been taken, with results expected Thursday afternoon.