An outbreak of acute gastroenteritis occurred in a primary school southwest of Sydney in August, 1977. Two hundred and eight pupils and nine teachers were affected over a one-month period. Electron microscopy revealed parvovirus-like particles in faecal extracts of 14 of 23 persons tested during the acute phase of illness. The virus was still present in the faeces of four individuals six weeks later. The appearance of specific antibodies was demonstrated by immune electron microscopy in five persons. No other cause for the outbreak could be found despite exhaustive investigations.