Theodore Roosevelt Beach in Oyster Bay, Laurel Hollow Beach in Laurel Hollow and Philip Healey Beach in Massapequa are all off-limits to swimmers.
The county health department says it will reopen the beaches as soon as water samples indicate the bacteria levels have dropped off.
The closings come as the summer weather begins to heat up in the Tri-State area.
In New York City, pool season officially reopened on Saturday.
Outdoor pools reopened at full capacity and will be available from 11:00 a.m. to 3:00 p.m. and 4:00 p.m. to 7:00 p.m., seven days a week.
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