Conjugal mating experiments were performed between donor strains of Vibrio cholerae carrying the vibrio sex plasmid P and recipient strains lacking the P plasmid. Donor and recipient genotypes differed with respect to toxinogenicity (tox), nutritional requirements, and antibiotic susceptibilities. Recombinants carrying selected donor and recipient markers were produced at low frequencies in conjugal matings. Mapping of tox markers was accomplished by scoring for the frequency of coinheritance of the donor tox allele with selected and unselected donor markers. Four independently isolated tox markers were analyzed. Each of these four tox markers was shown to be linked to the his-1 site in linkage group I on the genetic map of V. cholerae. In matings between a recipient strain carrying the tox-1 marker and donor strains carrying either tox-2, tox-3, or tox-4, all selected his+ recombinants remained nontoxinogenic. Matings between multiply marked strains demonstrated that the position of tox-1 with respect to other genetic loci in linkage group I is as follows: met-2--trp-1--asp-1--nal-1--his-1--tox-1. These findings demonstrate that a chromosomal determinant linked to his-1 in linkage group I on the genetic map of V. cholerae is essential for toxinogenesis and suggest that tox-1,tox-2, tox-3, and tox-4 may be alleles of a single tox gene.