Analysis of electrophoretypes of RNA of rotavirus which had circulated in Moscow and Leningrad in the winter of 1987-1988, detected by enzyme immunoassay (EIA), was carried out. RNA electrophoresis was performed in 10% polyacrylamide gel (PAG) followed by silver staining, Most of the strains isolated in Moscow and Leningrad had a long phoretype (67% and 77%, respectively. The greatest variations in PAG mobility were found in segments 2, 3, and 7-9, segments 1, 4, 10, and 11 showed most unchangeable mobility. According to the pattern of segment migration, 3 variants of the long phoretype and 5 variants of the short phoretype were distinguished. In Moscow the ist variant (confluent 2nd and 3rd segments as well as 7th and 8th segments) of the long phoretype was predominant, which was isolated in approximately 70% of all cases of infection; in Leningrad the dominating variant was intermediate between the 1st and 2nd phoretype (slower migration of the 2nd segment). Variants of the sport phoretype were characterized by greater variability and lack of the dominating strain. The potentials of rotavirus RNA electrophoresis as a method of molecular epidemiology are discussed.