Mild depletion of thymocytes, with swelling of thymic lymphoblasts was observed 1 week post inoculation (pi). The affected cells had irregular nuclei and frequently contained intranuclear inclusion bodies consisting of fine granular or homogenous materials. Electron-opaque regions, microtubules and autophagosomes were also present in the cytoplasm of affected cells. Aggregation of virus-like particles of 14 to 18 nm in diameter was rarely observed in the degenerative cells. The reticular cells with elaborate cytoplasmic meshwork were prominent 2 weeks pi. Repopulation of thymic lymphoblasts was marked in the cortex 3 weeks pi. The fine structure of the thymic cortex is also described.