We have mapped and determined the nucleotide sequence of the cowpox virus (CPV) gene coding for the protein of A-type inclusion bodies (ATI). The ATI gene was mapped to the central part of the largest HindIII restriction fragment of the CPV genome. Vaccinia virus (VV) also contained a similar sequence at the equivalent position in its genome, which even under stringent conditions hybridized with the CPV ATI gene. Nucleotide sequence analysis revealed that the upstream region of the coding frame contained a sequence closely resembling the VV late promoter consensus sequence and that the gene encoded a protein of Mr 150,000.