Over the last 5 years, there has been an increasing interest in the genetics of organisms other than the Enterobacteriaceae. The success of the genetic study of Escherichia coli has persuaded microbiologists, biochemists, and mo-lecular biologists that solutions to theoretical and applied problems in biology can be success-fully approached by understanding genetic phe-nomena in various microorganisms. Among those organisms which have attracted increasing attention are members of the genus Pseudo-monas, and the features which enticed earlier workers to the genetic study of this genus remain of interest. They include its notable biochemical diversity, gnificance for infectious disease, par-ticularly in terms of its resistance to antibiotic therapy, and increasingly the realization that there are genetic phenomena in this genus which are different from those in other bacterial gen-era. There have been previous reviews on the genetics of Pseudomonas (67, 69, 76, 77, 172). With respect to the biochemical diversity of Pseudomonas, considerable attention has been paid to theidentification and characterization of plasmids which carry genetic determinants that 73 enable utilization of a wide variety of growth substrates; this aspect has been recently re-viewed by Chakrabarty (20). Knowledge of these plasmids and their contribution to the understanding of the genetics and biochemistry of Pseudomonas requires a background of intensive and extensive information of the whole genome. Forexample, it is necessaryto be able to differentiate accurately between a function coded for by a chromosomal gene and a function coded for by a plasmid gene. Much of the sophisticated genetic work on E. coli depends on detailed mapping of the chro-mosome, which has proceeded for over 30 years. There is no reason to believe that progress in genetic studies of other bacteria will be satisfac-tory without extensive chromosomal mapping data. This article reviews recent studies on the bacterial chromosomal genetics of Pseudomonas species.