The sex ratio of fleas and their phoretic mites associated with the Nile grass rat, Arvicanthis niloticus (Desmarest), were studied during 14 mo in a grassland community of Lake Nakuru National Park, Kenya. Females of the fleas Dinopsyllus lypusus Jordan & Rothschild, Ctenophthalmus calceatus cabirus Jordan & Rothschild, and Xenopsylla cheopis bantorum Jordan infested more grass rats and in greater numbers than did males. Phoretic hypopi (hetermorphic deutonymphs) of two species of mites, Psylloglyphus uilenbergi Fain and Paraceroglyphus xenopsylla Fain & Schwan, varied seasonally in their abundance on fleas and utilized female fleas over male fleas for their major source of transport. Additionally, the mites were very host specific with nearly 100% of those identified on D. lypusus and C. calceatus cabirus being P. uilenbergi and 89% of the mites identified on X. cheopis bantorum being P. xenopsylla. This level of specificity suggests that these mite-flea associations are highly evolved. The importance of female fleas as hosts for transporting mites also suggests that female-biased sex ratios of fleas on their hosts may be caused, in part, by females being more important as dispersers within flea populations.