A tissue cyst-forming coccidian morphologically resembling the known species Hammondia has a mink-muskrat life cycle. Cysts are found in skeletal muscle of muskrats (Ondatra zibetheca). Mink (Mustela vison) fed infected muskrat carcasses shed oocysts for 4 to 6 days after a prepatent period of 6 to 8 days. The oocysts, 99% of which are unsporulated in mink feces, measure 11.5 to 12 μm × 10 to 11 μm. Sporulated oocysts have 2 sporocysts, each with 4 sporozoites. The present work was insufficient to establish whether this Hammondia-like parasite is identical to the known Hammondia spp. or is a new parasite, although the evidence gathered supports the hypothesis that this parasite is a new member of the genus Hammondia.