A critical vulnerability in the life cycle of freshwater mussels is a period of obligate, larval parasitism on fish. Descriptions of mussel-host relationships are often based on laboratory studies that develop a list of host fishes physiologically capable of bearing glochidia (larval mussels) to metamorphosis. Effective conservation of unionid mussels relies on understanding this relationship. To better understand this relationship, we studied host infestation by an endangered mussel, Popenaias popeii, in the Black River, Eddy County, NM. Field sampling revealed substantially fewer fish species (5) infested than predicted by a previous laboratory study (25) where fishes were exposed to glochidia. Bottom-dwelling suckers (Catastomidae) exhibited the highest infestation rate (80% for Carpiodes carpio) and the highest number of infected individuals; substantially fewer individuals (proportionally and total) were infested in other families. Suckers accounted for 64% of all infested fishes and bore the greatest number of glochidia (up to 1330). This suggests that not all hosts identified in the laboratory were ecologically relevant. We suggest that field studies of prevalence and intensity, along with lab studies of glochidial transformation success, be used to evaluate the relative contributions of fish hosts to mussel recruitment. Such studies will identify host species that are essential for maintaining healthy populations of freshwater mussels.