Freshwater mussels are one of the most endangered taxa in North America.  Their complex life histories, including obligate, larval parasitism on fish, exacerbates this condition.  Effective conservation of unionid mussels relies on understanding this relationship, including host specificity and prevalence of infestation.  To elucidate the nature of this relationship, we studied glochidia (larvae)-host relationships of an endangered mussel, Popenaias popeii, in the Black River, Eddy County, NM.  Because this is the only unionid in this river, infestation was unambiguously attributable to P. popeii.  We surveyed fish throughout the range of P. popeii from 2005-07.  Substantially fewer fish species (≤5) were infested than predicted by a previous laboratory study (25), where fish were experimentally exposed to glochidia.  Catastomids exhibited the highest infestation rate (e.g. 80%, Carpiodes carpio) and the highest number of infected individuals (8); substantially fewer individuals (proportionally and total) were infected in other families.  Therefore, we hypothesize that catastomids contribute more to the recruitment of P. popeii than other fish species previously identified as suitable hosts.  Field observations suggest that this mussel takes advantage of catastomid behavior.  Mussels release glochidia in a net-like structure, which may facilitate exposure to catastomids that frequently visited mussel aggregations.  Glochidia were encysted primarily on the face of fish rather than on the gills, as is common in other other unionids.  Our preliminary results suggest not all hosts identified in the laboratory study were ecologically relevant and P. popeii may exhibit infestation strategies that take advantage of the behavior of their hosts.