The electronmicroscopic examination of stool samples from 18 infants and young children with gastroenteritis, hospitalized at the Clinic of Infectious Diseases in Prague, was carried out. In ten children rotavirus was found in the faeces and the bacteriological findings were negative. Rotavirus particles were aggregated by convalescent child sera and by normal human gamma globulin. The clinical picture was characterized by the sudden onset of vomiting and fever, in one case with febrile convulsions. Diarrhoea was watery and yellow-green, and usually persisted for four to five days. The condition of the children improved rapidly after rehydration and a special diet. Older siblings, and in one family also adult members, were frequently affected.