SEXUAL recombination in Pseudomonas aeruginosa was first reported by Holloway1. Later, Holloway and Jennings2 demonstrated that there were similarities between their mating system and that of Escherichia coli K12. Fertility in the males was associated with an infectious factor which Holloway and Jennings called FP to distinguish it from the fertility factor, F, in E. coli. They were, however, unable to free their cells from the FP factor with acriflavine. Holloway1 reported 125 recombinants per 109 parent cells.