Among members of 33 families, 37% had antibodies to coxsackievirus A9 before, and 93% after, an outbreak of this disease. In all, 88% of the susceptible members of this population converted, serologically. Rash was age related, being more frequent in the very young. Clinical illness was mild but expressed more often in the very young. Among 480 children less than 6 years of age who were bled in well-baby clinics after the epidemic, 58% were found to have coxsackievirus A9 antibodies. Coxsackievirus A9 was the most frequent cause of aseptic meningitis in the community during the epidemic period.