The results of screening of 510 patients with acute gastrointestinal diseases over the period of 1986 to 1991 are presented. Rotaviruses were shown to be the infection agents in 112 (60%) children and 76 (40%) adults. The phoretyping of the isolated strains revealed cocirculation of 17 different phoretypes of rotaviruses: 8 "long" and 9 "short" ones. At the same time, two predominant phoretypes were found to alternately circulate in various epidemic seasons. The "long" phoretypes were found to be equally prevalent among adults and children, whereas the "short" phoretype rotaviruses were isolated mainly from children. The cyclic pattern of circulation of rotavirus phoretypes and genetic changeability of these viruses as a factor determining the prevalence of rotaviral infection are discussed.