A total of 1,021 freshwater mussels (Bivalvia: Unionidae) representing 12 species collected from 23 rivers between May 2002 and September 2006 in Manitoba, Saskatchewan, and North Dakota, U.S.A. were examined for aspidobothrean parasites. Nine of the 12 species were infected with aspidobothrean parasites. Anodontoides ferussacianus, Lasmigona complanata, Lasmigona compressa were uninfected. Aspidogaster conchicola was recovered from 77 individuals representing 6 of 12 mussel species. Prevalence as high as 100% was recorded for Amblema plicata and Potamilus alatus. The highest infection intensity was 254 observed in P. alatus. Cotylogaster occidentalis was recovered from 91 individuals representing 8 of 12 mussel species. Prevalence was typically lower by comparison with A. conchicola, although samples of single infected Fusconaia flava, Lampsilis siliquoidea, and Strophitus undulatus resulted in 100% prevalence in some localities. Cotylogaster occidentalis infection intensity was also much lower with the highest being 10 from Lampsilis cardium. This represents the first report of A. conchicola and C. occidentalis from Manitoba and North Dakota. Aspidogaster conchicola is reported for the first time from Lampsilis cardium. Cotylogaster occidentalis is reported for the first time from the mussels Amblema plicata, Fusconaia flava, Lampsilis cardium, Potamilus alatus, Quadrula quadrula, and Strophitus undulatus. Aspidogaster conchicola was not found infecting some mussel species that had been previously reported as parasitized hosts, perhaps suggesting local host specificity.