The effect of the age of adult Echinostoma caproni on egg development was studied. The percentage of fully developed miracidia was determined in eggs derived from adult worms obtained from laboratory mice at 2, 4, 6, and 8 wk postinfection (PI). Regardless of the age of worms from which the eggs were obtained, the percentage of fully developed miracidia was always >90%, and 60–80% of the eggs hatched. Several previous studies have shown that eggs derived from 2- to 4-wk-old E. caproni yielded miracidia that infected Biomphalaria glabrata snails. Results of the present study on E. caproni were in marked contrast to previous results with Echinostoma friedi, for which viable eggs were not obtained at 2 and 3 wk PI and maximal infectivity of miracidia in snails was obtained from eggs derived from worms collected at 8 and 9 wk PI. Further studies are needed to determine if the egg viability of other species in the “revolutum” group follow that of E. caproni or E. friedi.