Infectivity, development, and growth of Zygocotyle lunata in 18 male Balb/c mice, each exposed to 20 metacercariae cysts, were examined 1 to 6 wk post-infection (PI). Metacercariae cysts were obtained from naturally infected Helisoma trivolvis snails. At necropsy, 11 of 18 mice were found infected with a total of 32 worms. The range of recovery was 1 to 6 worms per host. The mean body area of worms, fixed in 10% neutral-buffered formalin and mounted in glycerin jelly, was found to be 4.0 mm2 at 2 wk PI, increased to 6.8 mm2 at 4 wk, and reached 11.4 mm2 by 6 wk PI, at which time the study was terminated. Worms were ovigerous by 3 wk PI. Eggs were collected from 4-wk-old worms and incubated at 28 C in artificial spring water (ASW). Eggs were fully developed by 3 wk post-incubation and miracidia hatched in the ASW. Laboratory-reared H. trivolvis snails were not available to attempt miracidia infections with Z. lunata.