Sexual stages of a Sarcocystis of passerine birds were demonstrated by feeding muscle cysts from cowbirds (Molothrus ater) and grackles (Cassidix mexicanus) to opossums (Didelphis virginiana). Opossums were examined at necropsy at 36 h, 13.5 days, and 29.5 days post-inoculation (PI). Large numbers of macro- and microgamonts were present in the small intestine 36 h PI, while only sporocysts and oocysts were present 13 days or more PI. Microgamonts in different stages of development were in the intestinal epithelium above the nucleus and macrogamonts were at the base of the epithelial cells or in the lamina propria. Oocysts and sporocysts were in the core of villus, beneath the basement membrane. Descriptions are given of the sexual stages as they appear in sections and smears.