Previous studies on rotavirus gene-coding assignments failed to clearly identify the simian rotavirus SA11 genome segment 3 protein product. This question was reexamined by using new conditions of electrophoresis with improved resolution of proteins in the high-molecular-weight range. Our results showed that the SA11 genome segment 3 codes for a protein with an apparent mol wt of 88,000. This protein normally comigrates with the protein product of genome segment 4. The gene 3 protein was located in viral core particles by comparing the electrophoretic patterns of [35S]methionine-labeled viral polypeptides from infected cells and from purified double-shelled, single-shelled, and core particles. To confirm the identity of the gene 3 product, we also studied two reassortant viruses in which genome segment 3 was reassorted from each of two parental virus strains (SA11 and NCDV). The gene 3 and gene 4 products of these viruses were identified by (i) their separation by two different polyacrylamide gel systems, (ii) their location in distinct particle types, (iii) their differential sensitivity to trypsin digestion, and (iv) their distinctive protease peptide maps. We propose that the genome segment 3 product be called VP3 and that the gene 4 product be named VP4 from now on.