The trematode Quinqueserialis quinqueserialis (Barker and Laughlin, 1911) was established in the laboratory in 18 rodent hosts. Sixteen of these had not been recorded previously as natural hosts. Growth and development was studied in Microtus montanus, Microtus pennsylvanicus, and Ondatra zibethicus. Growth was most accelerated in M. montanus (prepatent period 15 days), slower in M. pennsylvanicus (prepatent period 18 days), and slowest in 0. zibethicus (prepatent period 28 days). In all hosts, growth was shown to continue considerably past the time of sexual maturity. Statistical analysis of worm populations of equal age from different hosts demonstrated a definite host effect on morphology. Specimens from voles of the genus Microtus were consistently larger than specimens from 0. zibethicus. Body length, body width, oral sucker width, width of the uterine coils, testis length, ovary length, cirrus sac length, and metraterm length were all found to be host-dependent to some degree. No significant differences in egg length were noted. On the basis of the variation noted and a study of comparative material, three species of the genus were considered valid: Q. quinqueserialis, Q. wolgaensis Skvortsov, 1935, and Q. floridensis Rausch, 1952. Increasing attention has been focused in recent years on the dramatic variations in a parasite's structure, physiology, and behavior resulting from its development in different host species. Watertor (1967), working with Telorchis bonnerensis Waitz, 1960, demonstrated a pronounced decrease in size and delayed development in generations raised in reptilian hosts compared to those raised in amphibian hosts. Similar variation in size with differing hosts was recorded for Schistosoma mansoni Sambon, 1907, by Bruce, Llewellyn, and Sadun (1961). During a survey of the helminth parasites of arvicoline rodents in western Montana from 1964 to 1966 (Kinsella, 1967), a trematode, Quinqueserialis quinqueserialis (Barker and Laughlin, 1911), was commonly found in the cecum of the muskrat, Ondatra zibethicus (L.), and the meadow vole, Microtus pennsylvanicus (Ord). Specimens from voles appeared to be larger and thicker than specimens from muskrats, a phenomenon which had been noted Received for publication 15 May 1970. * Part of a dissertation submitted to the Graduate School, University of Montana, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Supported in part by an NIH predoctoral fellowship, 5-F1-GM-32, 786-02. t Present address: Department of Veterinary Science, University of Florida, Gainesville, Florida 32601. previously by several authors (Edwards, 1949; Rausch, 1952). The purpose of this investigation was to establish the life cycle of Q. quinqueserialis in various hosts, eliminating as much as possible other sources of variation, such as parasite age, host age, and host diet, in order to evaluate the effect of the host on the development of the parasite. MATERIALS AND METHODS Eggs of Q. quinqueserialis used for experimental feedings were originally obtained from naturally infected muskrats live-trapped in the vicinity of Missoula, Montana. Fecal material was thoroughly mixed in distilled water and filtered through 4 layers of cheesecloth. After centrifugation at 1,500 rpm, the supernatant was poured off, and the resulting plug was broken up in 5 ml of distilled water. One or 2 drops were counted to obtain an approximate number of eggs per drop. The molluscan intermediate host, Gyraulus parvus Say, was collected from flooded gravel pits along Miller Creek, Missoula County, Montana. Snails were maintained in enamel pans containing pond water and washed sand, and were fed on leached maple leaves. At the time of infection, snails were placed in a large petri dish and enough of the fecal suspension added to give 4 or 5 eggs per snail. Snails were exposed to infection in the dishes for several days and then returned to the enamel pans. When snails were maintained at 25 C, cercariae first emerged at 28 days postinfection and large numbers were produced by days 30 to 35. Snails kept in total darkness did not release cercariae. To