Distribution of two ecologically similar but usually spatially separate ectosymbionts, branchiobdellidans (Annelida) and scutariellids (Platyhelminthes), on atyid shrimp (Neocaridina spp.) is reported from 18 localities in five Provinces of southeastern China. Prevalence was determined for the branchiobdellidan, Holtodrilus truncatus, found at seven locations, the scutariellid, Scutariella japonica, present at every site, and where cohabitation occurred. Both ectosymbionts showed a microhabitat predilection for the host's branchial chambers and instances of cohabitation occurred at all seven locations where H. truncatus were collected, although not on every shrimp. On-site observations of live hosts supporting both ectosymbionts showed that neither H. truncatus nor S. japonica reacted aggressively or defensively towards the other when in close proximity. Instances of imported Chinese Neocaridina spp. into central Honshu Island, Japan, almost certainly came from areas in southeast China identified in this study. These imported populations are predicted to spread northwards into the area where endemic Japanese branchiobdellidans occur.