In 1984, the incidence of shigellosis was 32.4 per 100,000 population in Pima County, Arizona. To investigate sources, Shigella isolates and epidemiologic data were collected for 79 cases of infection with Shigella sonnei, the most common species. Since S. sonnei has a single serotype, plasmid isolation was attempted to refine the epidemiologic analysis. There were seven plasmid patterns containing 17, 13, 4, 22, 9, 2, and 3 isolates. Twelve of 17 isolates associated with recent travel to Mexico were in a group distinguished by a 5.1-kilobase (kb) plasmid. This plasmid was used to probe Southern blots of plasmids from strains of all groups. The Mexico-related plasmid probe hybridized to all the 5.1-kb plasmids and to 5.5- and 7.4-kb plasmids from three other groups. Of the 79 isolates, 50 contained plasmids homologous to the Mexico-related plasmid probe, suggesting association with travel to Mexico.