After intensive testing throughout the weekend, Lambert’s Cove Beach and Long Point Wildlife Refuge Beach in West Tisbury remained closed to swimming on Monday afternoon due elevated levels of enterococcus. All beaches in Oak Bluffs had opened by Saturday afternoon, except for Madeiros Cove, which remains closed.

A large number of beaches in the two towns were closed to swimming last week. The last several days of testing have yielded lowered results in all samples, but the bacteria levels need to come under a geometric mean of 35 parts per million, said Chris Kennedy, the Martha’s Vineyard Superintendent for The Trustees of Reservations. “We were running samples all weekend,” said Mr. Kennedy. “The last five samples range from 2 to 2,420, which is what forced the closure. The mean of the last five samples has to be 35 or less, and we’re at 47 now.”

West Tisbury health agent John Powers echoed Mr. Kennedy’s remarks yesterday. “We’ve been testing daily since July 14. We tested all weekend, and again today,” he said. “We’re close — we’re hoping that we’ll be within the geometric mean in the next day or two. Yesterday’s results are low — they’re down under the mean, but we need five consecutive test results that meet the lower mean.”

For the most part, beachgoers appear to have been obeying the warnings. “We have the beaches posted, but what I’ve told many people is that we aren’t beach cops,” Mr. Kennedy said. “The waters are administratively closed, but the most recent samples are well under. We’re still telling people to use their best judgment.”

So far, no illness has been reported in connection with the high bacteria counts in the water. Enterococcus bacteria can pose a health risk to young children, the elderly and people with compromised immune systems.

If it rains again, the opening of the beaches could be further delayed. “If we have rainfall, we have to wait 24 hours to resume testing. We’re keeping our fingers crossed that it doesn’t rain so we can still test and then open up the beaches,” said Mr. Kennedy.

Meanwhile, the source of the bacteria remains unclear. “We’re looking at some of the runoff from Coca Cola Brook [a freshwater stream that runs onto Lambert’s Cove Beach] and James Pond,” Mr. Powers said. “We haven’t pinpointed a source, but it could have happened last week after the heavy rains, we just don’t know.”

The heightened levels could also be a sampling or laboratory error, perhaps a result of a misreading or a dirty sample bottle. “I’m highly suspect of that number, but we have no leeway and we have to test,” Mr. Kennedy said. “Common sense tells me that it was most likely an anomaly. But until we have newer samples, that high number is still part of our mean.”