Electrophoresis on 5% polyacrylamide was used to analyze dsRNAs of 26 cucumber mosaic virus isolates propagated in Nicoîiana tabacum. There was variation between isolates in the migration of each of the dsRNAs 1,2, and 3. Comparison of the dsRNA profiles enabled each isolate to be allocated to 1 of 7 distinct dsRNA profile types. Two distinct and readily distinguishable isolates were mixed in planta and dsRNA from these infected plants compared with in vitro mixtures of them. All bands from both types were present, indicating that the differences were real and reproducible. This method is of value as a means of classifying cucumber mosaic virus isolates, as it more closely reflects a range of biological characteristics than do other methods currently used