Freshwater unionid mussels are among the most endangered groups of organisms in the world. They develop indirectly via a host, usually a fish, and this dependence appears to limit the reproduction and distribution of freshwater mussels. Epioblasma torulosa rangiana and Lampsilis fasciola are 2 endangered species in Canada. Epioblasma t. rangiana has a low abundance and limited distribution, whereas L. fasciola has a higher abundance and less-constrained distribution. Three known host species were examined for each mussel species. Results were that: 1) E. t. rangiana glochidia had significantly higher metamorphosis rates (i.e., proportion of attached glochidia that successfully metamorphosed to the juvenile mussel stage) on Etheostoma exile (mean ± SE: 44 ± 9%) and Cottus bairdi (42 ± 6%) than on Etheostoma nigrum (10 ± 3%) and 2) L. fasciola glochidia had significantly higher metamorphosis rates on Micropterus dolomieu (82 ± 2%) and Micropterus salmoides (63 ± 8%) than on C. bairdi (37 ± 7%). Variation in the co-occurrence of mussels and their primary vs marginal host species (i.e., high vs low infestation and metamorphosis rates, respectively) appears to explain the distributions and abundances of these 2 endangered mussels in Canada. An understanding of the quality of different host fishes of endangered mussel species is needed to facilitate effective conservation strategies.