The seasonal population dynamics of Halipegus occidualis and Halipegus eccentricus (Hemiuridae) in their amphibian host, the green frog (Rana clamitans), were examined for 3 yr (weekly, April through October). Frogs were caught, marked, and examined for H. occidualis and H. eccentricus, both of which occur as immatures and adults in the buccal cavity. Frogs were then released and allowed to continue natural recruitment and loss of the parasites. It was thus possible to monitor individual infrapopulations over successive time periods. One-hundred and forty-nine frogs were caught and released, with a total of 328 observations. Overall, the levels of both parasites among male and female frogs were similar, as were levels of infection among adult and juvenile hosts. There was, however, no correlation between the total number of H. occidualis and the body size (snout-vent length) of R. clamitans. In contrast, there was a significant correlation between the total number of H. eccentricus and frog size. Recruitment of both species began in May, peaked in June/July, and ended in July (H. eccentricus) or August (H. occidualis). In general, both the prevalence and relative density of H. occidualis was greater than that of H. eccentricus and may be related to space constraints in the buccal cavity of R. clamitans. Adults of both species were observed from April through October. Following increases in parasite recruitment, infrapopulation sizes declined in September 1992 and in August of 1993 and 1994. Examination of variance to mean ratios indicated that both species were overdispersed in the frogs. Large declines in the variance to mean ratios for H. occidualis after periods of greatest recruitment are most likely associated with the loss of larger infrapopulations, suggesting that there may be density-dependent regulation of infrapopulation size. By monitoring individual hosts using the mark-release-recapture protocol, dynamic changes in parasite infrapopulations were observed, e.g., there were losses of immature worms and rapid changes in infrapopulation sizes, observations that would not be made with typical host-parasite systems.