Inkwell Beach in Oak Bluffs was closed to swimming Wednesday after water tests showed unacceptable levels of bacteria. Signs were 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 around the Vineyard. Inkwell and several other beaches were closed temporarily just two weeks ago, in the height of the summer tourist season as well.

The state Department of Public Health is doing an investigation of the issue of why bacteria readings were so elevated across the Massachusetts seaboard this summer, Mr. Caron said, although a spokeswoman at the department was unable to confirm that.