Although there are several similarities in the life cycles of the azygiid trematodes Leuceruthrus micropteri and Proterometra macrostoma, their prevalence in their snail intermediate host and infection intensity in their centrarchid fish definitive hosts at North Elkhorn Creek, Kentucky, U.S.A., are markedly different. Here, we provide evidence for factors affecting their infection intensity in centrarchids based on (1) duration of cercarial swimming, (2) conditions and time required for release of the distome body from the cercarial tail chamber in the fish stomach, and (3) possible differences in the developmental rate of worms in a common definitive host. Cercariae of P. macrostoma swam considerably longer than those of L. micropteri; no cercariae of L. micropteri were observed swimming off of the bottom of beakers after 8 hr, whereas 67% of the P. macrostoma cercariae were still swimming after 13 hr at 20°C. Almost all of the P. macrostoma distomes were released from their cercarial tail chambers in vitro after 60 min at pHs 1.5, 2.0, and 2.5 with and without 0.5% pepsin. Similarly, nearly all L. micropteri cercariae were released from their cercarial tail chamber at pH 1.5 with and without pepsin; however, they did not emerge at pHs 2.0 or 2.5 without pepsin. Although the emergence of the L. micropteri distome was stimulated by the addition of pepsin in the higher pH values, it was markedly slower than that of P. macrostoma. By contrast, in experimental infections of largemouth bass Micropterus salmoides at 24.6°C, all distomes of L. micropteri and P. macrostoma completely emerged from their tail chambers and were attached to the stomach mucosa of their bass hosts after 20 min postinfection (PI). Additional experimental infections of largemouth bass showed no increase in body length or width for P. macrostoma distomes on day 12, 21, or 31 PI, but an increase was apparent in the body length and width for L. micropteri distomes on days 21 and 31 PI at 24.6°C. No egg and only anlagen of testes and ovary were observed in L. micropteri by day 31 PI, whereas P. macrostoma adults contained fully mature eggs with miracidia by this time.