An outbreak of cryptosporidiosis occurred among visitors to a state park in New Jersey. Swimming in the lake was strongly associated with illness. The lake was likely contaminated from rainwater runoff and infected swimmers. Lake water samples showed the presence of Cryptosporidium and high fecal coliforms. A back up of the septic tank occurred in mid-July which resulted in an above ground sewage outflow. The next day there was heavy rain. This was the first known reported outbreak of cryptosporidiosis involving recreational water.