Black‐head, blunt‐nosed, and other minnows were found infested with encysted flatworms in ponds at Leetown, West Virginia. The mortality in a 2.2‐acre pond stocked with 100,000 black‐head minnows was about 250 per day during four weeks of observation. Heavy infestations caused sterility in the minnows. The cyst and parasite are described briefly and the probable life cycle given, followed by certain recommendations for the control of the infestation.