Eight hundred and forty-nine and 1,503 Opisocrostis bruneri (Baker) were removed from Spermophilus franklinii (Sabine) in 1982 and 1983, respectively. Two discrete peaks in mean intensity of infestation were observed, one in early May and one in late August to early September. Overall prevalence of O. bruneri on S. franlinii was > or = 0.75. The observed biweekly prevalence on adult male and female squirrels was always > or = 0.73 and > or = 0.67, respectively. All juvenile squirrels were infested. Female fleas with undeveloped ovarioles, and with or without sperm in the spermatheca, were predominant during the first 6 wk that squirrels appeared above ground after hibernation. Parous females predominated thereafter. Peaks of nulliparous female fleas were observed in early May and early July. Female fleas were favored in each trapping period except during a 2-wk period from the beginning of May to the end of June. The mean biweekly sex ratio (male/female) of fleas removed from S. franklinii was 0.70 and 0.72 in 1982 and 1983, respectively. Oogenesis in O. bruneri was not dependent upon the estrous cycle of female S. franklinii. It appeared that O. bruneri completed at least two generations per year in Manitoba.