A range of recombinant DNA techniques now enables whole genome analysis of any bacterium to be carried out without recourse to the classical means of bacterial genetic exchange. Using enzymes which cut infrequently, such as SpeI, combined with pulsed field gel electrophoresis, a physical map of ordered fragments can be constructed. By means of cloned fragments of known genes or oligonucleotides synthesized using data from DNA or protein sequence banks, the location of individual genes on this map can be determined. We have used these techniques to study whole genome structure in three species of Pseudomonas: P. aeruginosa, P. putida and P. solanacearum.