The life history of Neoechinorhynchus rutili (Mueller, 1780) was demonstrated experimentally. Shelled embryos are released by the adult worm into the lumen of the digestive tract of freshwater fishes and subsequently pass out with the feces. These "eggs" are ingested by ostracods (Cypria turneri). Within the intestine of the ostracod the acanthor hatches, penetrates the wall of the intestine, and in 6 to 12 days is found free in the hemocoel where it metamorphoses through the acanthella to the juvenile stage in 48 to 57 days. This stage is infective to the fish. The development in the ostracod has been followed and comparisons have been made with the other two species of the genus which have been studied.