Two species of mites (Astigmata: Acarina) were found crawling on the integument of cultured Nephelopsis obscura, a common North American leech. This is the first record of an association between mites and adult annelids. Histiostoma anguillarum (Histiostomatidae), previously described as an associate of farmed freshwater eels, were recorded in densities of 1–17 mites per leech, while an undescribed member of the genus Schwiebea (Acaridae) occurred only sporadically. Because Histiostoma and Schwiebea are typically filter-feeders and detritivores, it is unlikely that either feeds on healthy leech tissues. Rather, because many leeches were parasitized by a water mould, Saprolegnia sp. (Protista: Oomycota), it seems more likely that the mites fed on the mould and on bacteria and protozans attracted to the dying annelids. The relationships between astigmatid mites, annelids and fish are critically assessed in light of these observations.