The course of a 300 metacercarial infection of ICR mice with Echinostoma caproni was examined over a 24-d period. Seven groups of 3 mice each were infected with 300 metacercariae of E. caproni and examined at 24 hr postinfection (PI), 4 d PI, and then at 4-d intervals to 24 d PI; however, as a result of the death of 4 mice prior to necropsy, the 20-d collection was abandoned. All mice were infected at the time of examination with worm loads ranging from 13.3% to 94.3% of the initial inoculum. The lowest recovery was at 24 hr PI with an average of 26.7% and the majority of worms located in the posterior one fifth of the small intestine. From 4–24 d PI there was no significant difference in recovery with means from 52.3% to 68.0% of the infective inoculum. Worms were congregated in the last two fifths of the intestine at all collection periods. None of the mice that died during the course of the experiment demonstrated unusual behavior prior to death. There was no significant difference in worm load between the 4 animals that died 13–21 d PI and those that were necropsied 16 and 24 d PI (P = 0.435). The results confirm that heavy infections of E. caproni mice can successfully produce large numbers of adult worms; however, outbred ICR mice demonstrate individual differences to these infections that cannot be explained at this time.