The stomachs of 141 (96.5%) of 146 snowshoe hares (Lepus americanus) collected in the area of East Corinth, Maine between the months of February, 1972 and February, 1973 were infected with Obeliscoides cuniculi. Nematodes were counted in 140 infected adult hares. The greatest intensity of infection occurred during the period from spring into summer (March to July) and at this time the populations of nematodes were mainly adults. During this period, female hares harbored significantly greater numbers of nematodes than males. Lowest numbers of nematodes were seen in late fall and early winter (October to December). The proportions of immature (fourth stage) nematodes in the populations increased appreciably from late summer on into winter and by December these constituted some 60% of the total population. A marked decline in immature nematode numbers was observed after February, with a concurrent increase in numbers of adults. A well-defined cyclical population fluctuation in natural infections of O. cuniculi was evident.