According to the municipality, elevated levels of the intestinal bacterium ecoli have been measured in the lake.
Gnesta municipality therefore advises visitors not to swim in the bathing water at Frösjöstrand.
The municipality will do a new sampling at the bathing site and the new result is expected to come this week.
The cause of the elevated bacterial level is unclear.
Also last year, bad bathing water was reported in Frösjön.
The investigation that was then started is still ongoing, according to the municipality.