Swimmers were allowed to return to the beach at Beltzville State Park in Carbon County on Friday afternoon.
Tests of water samples from the lake showed the level of bacteria to be safe for swimmers, said the Pennsylvania Department of Conservation and Natural Resources.
The lake had been off limits to swimmers since Tuesday evening because of an unacceptable level of bacteria in the water, officials said.
The park has remained open for other recreational activities, including picnicking, hiking, boating and fishing.
For the latest information, contact the park office at 610-377-0045 from 8 a.m. to 4 p.m. daily.