Shigella are Enterobacteria comprising four different species, including Shigella sonnei. Their reservoir is strictly human, and transmission occurs via the fecal-oral route. They are extremely contagious, and the most common mode of transmission is person-to-person. On December 24, the coordinating physician at the Specialized Medical-Educational Institute (IMES) in Proisy, which provides care and specialized education to children aged 3 to 20 with severe disabilities, primarily intellectual, reported to the Aisne Departmental Directorate of Health and Social Affairs (DDASS) that between December 14 and 20, four cases of S. sonnei infection had occurred among the children at the institute. An investigation was undertaken to describe the temporal and spatial evolution of the outbreak, in order to identify the transmission pattern(s) and source(s) and to propose appropriate control measures. The investigation was retrospective and descriptive. It began in March 1999. This article presents the results of that investigation.