The generalized transducing phage Pf16h2 has been used to confirm linkage relationships of chromosomal markers of Pseudomonas putida previously determined from their time-of-entry in Hfr crosses, and to map new auxotrophic mutations. By means of spot matings using Hfr donors of known origin of transfer, catabolic markers forming part of a closely linked group of operons referred to as a superoperonic cluster have been shown to be chromosomally located and their map positions determined. R-prime-mediated interspecific complementation has been used to equate functionally 21 auxotrophic loci in P. putida and P. aeruginosa, and the distribution of these loci on the two genetic maps has been compared. While both maps reveal that auxotrophic markers are largely restricted to about 40% of the chromosome and that auxotrophic markers of similar phenotype are not clustered, there is evidence of at least seven chromosomal rearrangements since divergence from a presumed common ancestor.