Mass occurrence of intestinal disorders caused by Yersinia enterocolitica simultaneously in two groups of children of under-school age is reported. The question of the source of infection and the routes of transfer are discussed and the various forms pointed out which Y. e. infection may take in under-school children within the framework of an epidemic occurrence in a group of children. The fact is emphasized that the results of serological examination using the method of indirect haemagglutination may be an important guide for the orientation in the epidemiological situation and, in some cases, also in making etiological diagnosis when the proof of Y. e. by culture presents difficulties.