- SUMMARY This study set out to determine whether the sex ratio of fleas collected from host bodies is a reliable indicator of sex ratio in the entire flea population. To answer this question, previously published data on 18 flea species was used and it was tested to see whether a correlation exists between the sex ratio of fleas collected from host bodies and the sex ratio of fleas collected from host burrows. Across species, the female : male ratio of fleas on hosts correlated strongly with the female : male ratio of fleas in their burrows, with the slope of the regression overlapping 1. Controlling for flea phylogeny by independent contrasts produced similar results. It was also ascertained whether a host individual is a proportional random sampler of male and female fleas and whether the sex ratio in flea infrapopulations depends on the size of infrapopulations and on the gender and age of a host. Using field data, the sex ratio in infrapopulations of 7 flea species parasitic on 4 rodent species was analysed. Populations of 3 species (Nosopsyllus iranus, Parapulex chephrenis and Xenopsylla conformis) were significantly female-biased, whereas male bias was found in 1 species (Synosternus cleopatrae). In general, the sex ratio of fleas collected from an individual rodent did not differ significantly from the sex ratio in the entire flea population. Neither host gender, and age nor number of fleas co-occurring on a host affected (a) the sex ratio in flea infrapopulations and (b) the probability of an infrapopulation to be either female- or male-biased.