To test the hypothesis that oogenesis in 4 species of fleas is dependent on the reproductive cycle of their monoestrous host Spermophilus richardsonii, 6,416 Oropsylla (Oropsylla) rupestris (Baker), 5,893 Oropsylla (Opisocrostis) bruneri (Baker), 813 Neopsylla inopina (Rothschild), and 223 Rhadinopsylla (Actenophthalmus) fraterna (Baker) females were collected from active hosts at 3 locations in Manitoba during 1987, 1988, and 1989 and dissected. Based on morphological changes in the ovaries, female fleas were categorized as immature, nulliparous or parous. Nulliparous N. inopina predominated in April and early May. Immature and nulliparous O. bruneri predominated throughout April and May. Nulliparous O. rupestris were most prevalent in early June but occurred sporadically throughout the season. Nulliparous R. fraterna predominated only in April. The percentage of mated females was usually correlated with percentage parity for all species. Oogenesis in each of the 4 flea species was apparently independent of the reproductive cycle of their host. Female fleas of all reproductive categories were frequently collected before, during, and following reproduction in their hosts.