Myxobolus ridouti n. sp. and M. ridgwayi n. sp. are described, respectively, from Pimephales notatus (Rafinesque) and Semotilus atromaculatus (Mitchill) in Algonquin Park, Ontario, Canada. Both are intracellular in striated muscles of the body flank. Spores of M. ridouti n. sp. are subcircular (9.5–10.5 μm long by 9.4–10.9 μm wide); those of M. ridgwayi n. sp. are oval (10.0–12.1 μm long by 9.5–10.5 μm). Both species have spores with a small, distinct swelling (1 μm) on the spore valve near the posterior sutural ridge, similar to that described for Myxobolus insignis Eiras, Malta, Varella, and Pavanelli, 2005. It looks to be non-nuclear in origin, apparently part of the internal face of a spore valve, and could be taxonomically useful. Partial sequence data of the 18S rDNA from both species are unique among those species in the genus that have been sequenced. The sequences have the greatest overall identity with various species of Myxobolus from cyprinids in North America, Europe, and Asia, with a constructed molecular phylogeny having the 2 new species forming a separate clade.

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