Strains of certain types of Group A hemoiytic streptococci (3, 6, 17, 19, 44, 46) were discovered to be naturally resistant to moderate amts. of sul-fadiazine. It is suggested that such organisms originated the epidemics of streptococcal disease among troops receiving sulfonamide prophylaxis, later becoming more resistant by mutation. No strains significantly resistant to penicillin were discovered.