Freshwater mussels play an important role in stream ecosystems but are declining worldwide. The California floater (Anodonta californiensis, Lea 1852), formerly widespread throughout the Pacific Northwest, is listed as a federal species of concern and a candidate species for state listing in Washington (WA). Because freshwater mussels are obligate parasites on fish, conservation and restoration efforts require understanding specific host fish species. We found a previously undocumented population of A. californiensis in the upper Yakima River Basin in WA and determined its host fishes through a combination of laboratory and field studies. Two fish species, speckled dace (Rhinichthys osculus) and torrent sculpin (Cottus rhotheus), were confirmed as hosts for A. californiensis. Two fish species, three-spine stickleback (Gasterosteus aculeatus) and redside shiner (Richardsonius balteatus), were identified as hosts for A. californiensis in the laboratory portion of this study. Our results fill a significant data gap for building a successful conservation and restoration program by extending the known range of A. californiensis within the Yakima River Basin, and by identifying a suite of suitable host fishes for A. californiensis, including two that had not been previously confirmed as hosts. Our results can be incorporated into a management approach that focuses on the conservation and recovery of not only A. californiensis, but its host fishes as well.