Some years ago it was not possible in spite of intensive bacteriological and virological work on the etiology of acute infantile gastroenteritis to find any agent that may readily be linked to this disease. Since five years now early sporadic reports about virus-like particles have proved worldwide to be correct and in the meantime have substantially been supported by new findings in that these particles are the main causative agent of infantile gastroenteritis. As part of our electron-microscopic studies some morphological characteristics are shown. The physico-chemical as well as structural properties of the new virus apparently justify its classification into the family of the reoviruses. Rotavirus positive stool specimens were found in 46% of the material sent in by paediatric clinics.