Summary: Genetic recombination has been shown to occur in Pseudomonas aeruginosa. Crosses between 18 biochemical mutants of 4 strains were studied and, although non-random segregation was found for non-selective markers, it has not yet been possible to draw conclusions concerning gene order or linkage. The progeny obtained from crosses tended to resemble one parent more than the other. The 4 strains of P. aeruginosa used were interfertile in certain combinations but self-sterile. There was no evidence for an infectious principle affecting fertility but fertility changes may occur amongst recombinant progeny.