Nycteribiids and streblids are obligate hematophagous flies restricted to bats. These flies are widely distributed throughout the Neotropical region, although little is known about their spatial distribution and patterns in parasitic associations in different environments. Studies assessing bat-fly interactions are mostly qualitative, but quantitative studies that allow testing and/or establishing more specific patterns for these interactions are still scarce. Here, we qualitatively and quantitatively assessed the bat-fly interactions in an environmental protection area in northeastern Brazil using prevalence, mean intensity of infestation, aggregation, and abundance indices. We captured 654 bats of 21 species and five families. Only 157 bats (24%) were parasitized by 351 flies of 19 species and eight genera. Among the most representative bat species (n ≥ 15), the host-parasite relationship between the white-lined broad-nosed bat (Platyrrhinus lineatus) and a streblid fly Trichobius angulatus had the highest prevalence (33.3%). The highest mean intensity of infestation recorded was for Trichobius joblingi on the Seba's short-tailed bat (Carollia perspicillata) (2.05 flies per host). The fly with higher values of aggregation was Strebla guajiro (D = 0.98) on C. perspicillata. Ninety-five percent of the fly species were highly host-specific (monoxenous). Regarding the mean abundance of flies, we found no differences between dry and rainy periods. No noticeable pattern for northeastern Brazil was observed when comparing our results with previous studies, except for specificity that was consistently high throughout the region. Our study raises questions about the pattern of fly parasitism on bats and may guide future research on this subject.