Bacteriophage T7 capsid protein 10B has previously been proposed to arise by a translational frameshift near the 3' end of the capsid gene 10A coding sequence, adding an additional 53 amino acid residues to the carboxyl-terminal end of the protein. Here we show by peptide mapping experiments as well as by direct partial sequence analysis of an overlapping "junction" peptide, that 10B is in fact related to 10A by a -1 switch in reading frame in a narrow region near the carboxy terminus of 10A. Peptide mapping experiments demonstrate that 10A and 10B have the same amino terminus as well as virtually identical methionine-labeled peptide maps. However, the predicted unique carboxyl-terminal peptide from 10B was also identified. An overlapping peptide was isolated from 10B which spans the junction region in which the proposed translational frameshift is thought to occur. Partial sequencing of this junction peptide confirms a -1 frameshift within the last few codons of 10A.