SUMMARYIn May 1980 an extensive outbreak ofgastroenteritis occurred in a private school in the city of Rio de Janeiro. Examination of faeces and paired sera showed that this outbreak was caused by both rotavirus and a virulent strain of Shigella sonnei.In the first 19 stool samples collected seven (37 %) had rotavirus only, six (32 %) had Sh. sonnei only, while four (21 %) had both agents. Examination of the second and third stool collections revealed only the presence of Sh. sonnei. The 18 paired sera showed seroconversion for rotavirus in four cases (22 %) and in seven cases (39 %) for Sh. sonnei. The overall attack rate ofthe disease was approximately 75 %, the nursery and kindergarten having higher attack rates. Students in all grades became sick at the same time, and the unimodal curve of the onset dates of symptoms indicates a common source outbreak. Evidence suggested a contaminated water supply.