Phaseolotoxin [N delta(N'-sulfo-diaminophosphinyl)-ornithyl-alanyl- homoarginine] produced by Pseudomonas syringae pv. phaseolicola, the bean halo blight pathogen, is a potent inhibitor of ornithine carbamoyltransferase (OCT). Inhibition of OCT in infected plants leads to chlorosis and growth inhibition. A genomic cosmid clone, pHK120, containing a 25-kb fragment of DNA from a wild-type strain of P. syringae pv. phaseolicola restores toxin production in Tox- mutants. Tn5 mutagenesis of pHK120 and marker exchange of pHK120::Tn5 plasmids in the wild-type strain resulted in the isolation of 39 chromosomal mutants that harbor Tn5 insertions at known positions. Toxin bioassays revealed that 28 of the mutants, with Tn5 insertions distributed throughout the insert of pHK120, were Tox-, indicating that a functional locus for toxin production in each mutant was inactivated. Complementation analysis was done by testing for toxin production strains that carried a genomic Tn5 at one location and a plasmid-borne Tn5 at another location (pair complementation). Pair complementation analysis of nine marker exchange mutants and a random genomic Tn5 mutant revealed that there are a minimum of eight toxin loci (phtA through phtH) in pHK120. Mutants carrying Tn5 insertions in the phtA, phtD, and phtF loci were complemented by deletion subclones containing fragments from pHK120; mutants carrying Tn5 insertions in the phtC locus were partially complemented by a subclone, and mutants carrying Tn5 insertions in the phtB, phtE, phtG, and phtH loci were not complemented by any of the available subclones. A comparison of the insert from pHK120 with that from pRCP17, a clone reported previously (R. C. Peet, P. B. Lindgren, D. K. Wills, and N. J. Panopoulos, J. Bacteriol. 166:1096-1105, 1986) by another laboratory to contain some of the phaseolotoxin genes and the phaseolotoxin-resistant OCT gene, revealed that the inserts in these two cosmids overlap but differ in important respects.