Rotavirus RNA was detected in nasopharyngeal washings, obtained from patients with rotavirus gastroenteritis (76.3% of cases) and gastroenteritis of unknown etiology (34.7% of cases), by the method of molecular hybridization with the use of RNA of monkey virus SA11, labeled with 32P in the process of T4-ligase reaction. In 100% of cases this detection correlated with the presence of lesions of the upper respiratory ways and was probably indicative of the rotavirus nature of the catarrhal syndrome in rotavirus gastroenteritis.