Electron microscopic observations were carried out on five HBsAg carrier chimpanzees infected with delta (δ) agent and two chimpanzees infected with human non-A, non-B hepatitis. The cytoplasmic tubular structures, which have been recognized in the liver of chimpanzees infected with human non-A, non-B hepatitis, were found also in the liver of HBsAg carrier chimpanzees infected with 5 agent. The quantity of the cytoplasmic structures in serial studies was associated with SGPT elevation rather than with expression of ± antigen in sera and liver tissues. This indicates that the cytoplasmic structures reflect a pathologic change of the hepatocytes in chimpanzees infected with ± agent or human non-A, non-B hepatitis. These and other similarities between the two agents suggest a similar nature.