Nine strains of enterotoxigenic Escherichia coli that were resistant to antibiotics were tested for their ability to transfer both antibiotic resistance and enterotoxigenicity to E. coli K12. All nine isolates transferred antibiotic resistance in bacterial conjugation experiments, and in seven of these matings enterotoxigenicity was also transferred. To determine whether the genetic information coding for the production of enterotoxin and antibiotic resistance was located on the same plasmid in these strains, phage P1 transduction experiments were performed. P1 lysates prepared from enterotoxigenic antibiotic-resistant transconjugants were tested for their ability to transduce both phenotypes. Only P1 lysates prepared from transconjugants of E. coli 719B5 (O18ab: H27) were found to transduce resistance to streptomycin, sulfisoxazole, and tetracycline, as well as the ability to produce heat-labile and heat-stable enterotoxin. The results of these experiments strongly suggest that the genetic data coding for antibiotic resistance and enterotoxin production in E. coli 719B5 are carried on the same plasmid replicon.