To control syphilis among prostitutes and seasonal farm workers in Fresno County, California, we developed a program of selective mass treatment of prostitutes. From August 1976 to July 1977, 512 women entered the program. Overall, 4.6 per cent of women treated prophylactically for syphilis were actually infected. Compared to cases in the previous year, infectious syphilis cases among prostitutes and seasonal farm workers decreased 51.3 per cent and 26.8 per cent, respectively.