We used null models to examine patterns of species co-occurrences in 59 communities of fleas parasitic on small mammals from 4 biogeographic realms (Afrotropics, Nearctic, Neotropics, and Palaearctic). We compared frequencies of co-occurrences of flea species across host species with those expected by chance, using a null model approach. We used 4 tests for non-randomness to identify pairs of species (within a community) that demonstrate significant positive or negative co-occurrence. The majority of flea communities were non-randomly assembled. Patterns of flea co-occurrences on the same host species indicated aggregation but not segregation of flea species (except for the flea community of Madagascar). Although only a small fraction of species pairs were associated significantly (264 of 10, 943 species pairs according to the most liberal criterion), most of these associations were positive (except for 2 negatively associated species pairs). Significantly associated pairs were represented mainly by non-congeneric species. The degree of non-randomness of the entire flea community was similar among biogeographic regions, but the strength of pair-wise association varied geographically, being the highest in the Afrotropics and the lowest in the European region of the Palaearctic.