In the course of a six‐month‐study of acute gastroenteritis in children of ages up to six years, a reo‐like virus was found in 54 per cent of the faecal specimens obtained at an early stage of the disease, using electron microscopy as screening test. By means of a concentrated complement fixation antigen, composed of a related calf diarrhoea virus cultivated in tissue culture, the rise in titre was found to be significant in 96 per cent of the patients whose faeces contained the reo‐like virus. Antibodies were present in the remaining 4 per cent without rise in titre. In 10 per cent of the cases with gastroenteritis infection was caused by adenovirus or Salmonella. A probable aetiological agent was found in 71 per cent of the patients. It applies to 33 per cent of all cases caused by the reo‐like virus that they were nosocomial i