Cysts on calanoid and cyclopoid copepods, previously reported as tumor-like anomalies (TLAs) in Lake Michigan and Europe, appeared briefly in Patterson Lake, a small Michigan inland lake. Cysts were rare (4% maximum) in samples collected on September 11, 1999, but appeared with high frequency on calanoid adults (49%) and cyclopoid nauplii (73%) in samples collected on October 16. By October 30, cysts were again rare (0.4% maximum). Cysts most commonly appeared on the lateral surface of the animal at the articulation of the 1^st and 2^nd prosomal segments and often consisted of herniated copepod tissues. Transparent, pyriform cysts co-occurred in low frequency with other types of cysts and are believed to be the trophomeres and gonomeres of ellobiopsid parasites. Histologic manifestations of cysts were diverse; herniations consisted of acellular yolk-like material and apparent host tissue, while cysts thought to be Ellobiopsis contained cells with different degrees of nuclear staining and unusual spherical bodies. Hernias were experimentally induced on live calanoid copepods by piercing the carapace with a fine needle, suggesting that ellobiopsid parasites may cause the hernias by puncturing the carapace of their hosts. Ellobiopsid parasites are common on marine crustacean zooplankton but have been recorded only once before in freshwater.