spp. are cyst-forming coccidian parasites with a broad host range, infecting various wild and domestic animal species. Northamerican opossums () are severely affected by the infection with . This study presents a case of infection with in a road-killed female southern black-eared opossum ( in Misiones, Argentina. Many 0.5-1 mm cysts were observed in several muscles and visceral organs and were microscopically identified in skeletal muscles, tongue, and heart. Histological analysis disclosed multiple spherical cysts with a myriad of bradyzoites like-cells and a well-defined cyst wall. A small number of degenerate and ruptured cysts, surrounded by mild to moderate inflammation were observed. Genomic DNA from an individual cyst and muscle was extracted and ITS1 marker and gene fragments from sarcocystid protozoa were successfully amplified by PCR and sequenced. The sequence exhibited 100% identity with sequences of and . Comparison of the complete ITS1 sequence (259 bp) revealed an identity of 99.2% with and 97.7% with This result together with the phylogeny positioning, suggest that the sp. in the present case differ from , being closely related with .