A preparation of simian rotavirus SA11 was shown to contain, in addition to the normal 11 genome segments, an RNA species with electrophoretic mobility slightly higher than that of segment 4. Limiting dilution passages allowed the separation of two virus clones distinguishable from each other by the electrophoretic mobility of that genome segment. Possible implications of this finding in virus behaviour and in the comparison of rotaviruses by RNA electrophoresis are discussed.