In 1983, a small outbreak of infections caused by a previously unrecognized multiply-drug-resistant Shigella flexneri 3a strain occurred on the Hopi Indian reservation. The index patient, a diabetic woman with recurrent Escherichia coli bacteriuria on prophylactic trimethoprim/sulfamethoxazole (TMP/SMZ) therapy, was hospitalized with concurrent E. coli urinary tract infection and shigellosis. Both E. coli isolated from her urine and S. flexneri isolated from her stool were resistant to ampicillin, carbenicillin, streptomycin, sulfisoxazole, tetracycline, and TMP/SMZ. Both isolates contained a 35-MDa plasmid transferrable to recipient E. coli, and transconjugates acquiring the plasmid from either donor strain also acquired resistance to the same agents. The number and size of fragments generated by plasmid digestion with DNA restriction endonuclease ClaI were similar. A review of clinical microbiology records showed that an E. coli strain isolated from the patient 3 w before the onset of shigellosis had identical antimicrobial resistance to the E. coli and Shigella isolated during the outbreak. These studies indicate that the index patient receiving prophylactic TMP/SMZ was the likely source of the R-plasmid for the outbreak strain of Shigella.