SUMMARY: Genes determining the flagellar antigens were introduced from Hfr Salmonella abony into F− Escherichia coli by sexual recombination. The gene H1 for the phase-1 flagellar antigen of Salmonella was found to be allelic to H, the gene for the only flagellar antigen of E. coli. The phase-2 antigen of Salmonella has no counterpart in E. coli; if the corresponding salmonella gene H2 is introduced in E. coli, diphasic strains are produced whose phase-2 antigen is that of the Salmonella parent; the phase-1 antigen can be that of either parent. In each case the diphasic strains undergo a true phase variation, with either the phase-1 or the phase-2 antigen being expressed at one time. The rates of this variation are comparable to those in the Salmonella parent.