In vitro cultivation of trematodes would aid studies on the basic biology of the parasites and the development of chemotherapies and vaccines. Our goal was to measure the in vitro survival and maturation of metacercariae of Microphallus turgidus under different culture conditions. Metacercariae of M. turgidus from grass shrimp (Palaemonetes pugio) were excysted and cultured in humidified air at 37 C in RPMI-1640 medium supplemented with 20% calf, chicken, or horse serum. Deposition of eggs was greatest in media containing horse or calf serum. Worms survived longest at 37 C, but did not produce greater numbers of eggs than worms cultured in RPMI-1640–supplemented horse serum at 42 C. Most eggs deposited in vitro (>80%) were normal in shape and, after incubation for 10 days at 30 C in brackish water, approximately 30% of them contained miracidia. Eighteen percent of hydrobiid snails (Spurwinkia salsa) fed these eggs shed cercariae 5–6 wk later. The cercariae were infective to grass shrimp (Palaemonetes vulgaris) and developed into metacercariae. This study is significant because it is the second instance in which a digenean, and the first time that a microphallid, has been demonstrated to develop in vitro from metacercariae into adult worms capable of producing infective eggs.