We studied the colonization-factor antigen I (CFA/I) fimbriae- and heat-stable enterotoxin (ST)-coding plasmid of enterotoxigenic Escherichia coli (strain H10407, serotype O78:H11) pathogenic for humans. With use of conjugal-transfer system of E. coli H10407 and transposon-labeling techniques, the virulence plasmid was shown to be transferable to many species of the family Enterobacteriaceae, including the enteropathogens, Shigella and Salmonella species, and the opportunistic pathogens, Klebsiella, Enterobacter, Citrobacter, Edwardsiella, Serratia, and Proteus species. The virulence plasmid-carrying transconjugants produced both CFA/I fimbriae and ST (exotoxin). Moreover, most of the transconjugants stably inherited the virulence plasmid, although plasmid stability was greatly dependent on culture temperature. Finally, administration of the virulence plasmid-carrying living bacterial cells of Klebsiella, Enterobacter, and Citrobacter species and E. coli K12 resulted in fluid accumulation in both infant-mouse and rabbit ileal-loop tests.