An unusual outbreak of herpes simplex stomatitis occurred in a group of 13 children 11 mo to 35 mo of age. Herpes simplex virus was isolated from eight of the ten children with clinically apparent illness. In all cases, the viruses which were isolated in HeLa cell cultures caused cytopathic effects identical to those of the proliferative strain of herpes simplex virus and characterized by rounding and clumping of cells without formation of syncytia. Serologic studies demonstrated a rise in serum-neutralizing antibodies in all eight symptomatic children whose sera were available, and no rise in the three asymptomatic children. This outbreak was characterized by its unusually high attack rate, the nature of the strain of the virus isolates, and the lack of in-apparent infection.