Infestation with ectoparasites is particularly common among terrestrial animals, with birds and especially mammals holding diverse keratophagous and blood-sucking arthropod communities. Among the latter, bat fleas (Ischnopsyllidae) are highly specialised insects, with all Palearctic species feeding exclusively on bats. Studies on the ecology of fleas parasitic on bats are scarce, most report only occurrence and host species. Using a large dataset of bats captured in Europe, we assessed the importance of host species, host ecology (roost type) and season in regulating the occurrences of Palearctic bat fleas. We recorded 12 species of bat-specific and two generalist flea species on bats or inside their roosts. Our results suggest that bat fleas show regular, but rare presence on hosts, with variable prevalences on the different host species. Crevice dweller bat species were significantly more frequent hosts of fleas, than cave-dwellers, while host roosting habits (solitary or aggregated in tight group) was an even more important determinant for flea infestation. Host sex determined flea infestation rates, with males showing higher prevalences, while females hosting higher individual numbers of parasites. Season also had a marked influence, with certain flea species showing strict seasonality and high host specificity, while a single generalist, widespread flea species was recorded year-round. On-host populations of fleas showed female-biased sex ratios, likely due to their higher energetic needs.