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Three of 6 female long-clawed shrews, Sorex unguiculatus Dobson, 1890, collected on the island of Hokkaido, Japan, were found to have unsporulated oocysts and sexual stages (both macro- and microgamonts) in varying stages of development of an unidentified coccidium in both lactating and non lactating mammary glands. Gamonts developed in the alveoli of the mammary glands, and oocysts were found in the lactiferous ducts and in pools of milk. Mature macrogamonts were 11.9 × 15.2 μm (10-14 × 14-20 μm), whereas completely developed microgamonts with many gametes were 14.8 × 16.8 μm (10-18 × 13-20 μm). Oocysts in tissue sections were 19.5 × 13.8 μm and had a smooth outer wall that was <1 >μm thick. Little histopathology was associated with the infections. Infected cells were enlarged and appeared cloudy, and in some areas there was leucocytic infiltration by macrophages, small and large lymphocytes, neutrophils, and eosinophils, No basophil was seen. We also found sections of a nematode, probably a Mammanidula sp., in sections of an active mammary gland in I of the shrews not infected with the coccidium.