A swimming advisory was issued for beaches at Crandon South and North Shore, after water samples showed high levels of fecal bacteria, the Florida Department of Health in Miami-Dade said.
The results of the sampling exceeded the federal and state recommended standard for enterococci, which is a bacteria commonly found in feces.
According to the DOH, the fecal matter may have come from "storm water run off, wildlife, pets, and human sewage."
Residents have also reported a strong smell, similar to rotten eggs, coming from Crandon North Beach.
The advisory issued recommends not swimming at these locations until further notice.
"Contact with the water may pose an increased risk of illness, particularly for susceptible individuals," the DOH said.
Enterococci are normally found in the intestinal tract of humans and animals and can cause diseases, infections or illnesses for humans such as urinary tract infections and, though less frequently, meningitis and pneumonia.