Following a picnic, nearly half of the persons who attended became ill. Sixty-three of 139 throat cultures were positive for group A beta-hemolytic streptococci. Forty-seven of the positive cultures were available for typing, 34 of which harbored the epidemic strain, group A beta-hemolytic Streptococcus type T-3/13/B3264. This contrasted with two of 204 controls who had not attended, but harbored the epidemic strain. Food preferences and illness rates implicated the potato salad, and from it the epidemic strain was isolated. The attack rate and case-to-infection ratio among those who attended the picnic was significantly greater for persons over 10 years of age. Prompt institution of chemoprophylaxis could possibly have prevented at least 50% of the cases.