Lesions associated with encephalomyocarditis (EMC) virus in the testes of Syrian hamsters were investigated. Histopathologic changes were first detectable by light microscopy at 3 days postinoculation (DPI). Immunohistochemically, virus antigens were detected in the cytoplasm of germ cells and Sertoli cells in some seminiferous tubules beginning at 2 DPI. The following ultrastructural changes were observed: 1) swelling of mitochondria and dilatation of endoplasmic reticulum in germ cells, 2) large number of residual bodies in seminiferous tubules, 3) aggregates of virus-like particles in the cytoplasm of degenerated cells and tubular lumen, 4) condensation of cytoplasm and dilatation of endoplasmic reticulum in Sertoli cells, and 5) degenerative changes in capillary endothelial cells.