The reproductive biology of the California floater (Anodonta californiensis), western ridged mussel (Gonidea angulata) and western pearlshell (Margaritifera falcata) was studied in the Middle Fork John Day River from May 2005 to July 2011. Anodonta californiensis was gravid from early May to late July. Mature A. californiensis glochidia were hooked, rust-colored, sub-triangulate, averaged 276 µm in length, and were similar in size to other glochidia of the genus Anodonta. Gravid G. angulata were found only in June. Mature G. angulata glochidia were hookless, white, sub-round, and averaged 171 µm in length, similar to the oval pigtoe (Pleurobema pyriforme). Margaritifera falcata were gravid in early May. Mature M. falcata glochidia were hookless, white, sub-round, and averaged 55 µm in length, a size and shape similar to the spectacle case (Cumberlandia monodonta). Anodonta californiensis glochidia were found attached to six wild-caught fish species from early June to late July. In contrast, G. angulata glochidia were found attached only to wild-caught sculpin (Cottus sp.) in late July. No M. falcata glochidia were observed on fish in the field. In laboratory experiments, speckled dace (Rhinichthys osculus) was a confirmed primary host and the longnose dace (R. cataractae), and margined sculpin (Cottus marginatus) served as unconfirmed primary hosts for A. californiensis. Margined and shorthead sculpin (C. confusus) were identified as unconfirmed primary hosts for G. angulata.