Twenty-four female ICR mice, 12 acclimated to a 12∶12 light–dark cycle and 12 to a 12∶12 dark–light cycle for 7 days, were each infected with 10 metacercariae of Echinostoma caproni. Infected mice were maintained on their respective lighting regimes for 28 days. Six mice (3 from each group) were necropsied at 4-hr intervals beginning at 0700 hr. The small intestine was removed, opened, and the position of individual worms and worm clusters was measured to the nearest 0.1 cm. Each intestine was subsequently divided into 20 equal segments and individual worms and worm clusters were assigned to the appropriate segment based on the original measurements. All worms were found in the posterior 55% of the intestine (ileum). All posterior segments (10–20), with the exception of segment 18, harbored at least 1 worm at some time. A Monte Carlo simulation of worm abundance in segments 10–17 over all time periods indicated a random distribution, while the same analysis of segments 10–20 indicated a non-random distribution due to large numbers of worms in segment 20 and to the absence of worms in segment 18. To analyze temporal changes in worm distribution, mice were grouped by time of necropsy as follows: night (1900 and 2300 hr), morning (0300 and 0700 hr), and day (1100 and 1500 hr). During the night and morning, E. caproni was heavily concentrated in segments 10–17 and, during the day, worms were located more posteriorly, with a heavy concentration in the last segment (20).