SUMMARY: Auxotrophic, phage-resistant and streptomycin-resistant mutants of Caulobacter were used to demonstrate genetic recombination in this genus. Forty of a possible 170 mating pairs gave recombinant frequencies of from 100 to 1000 times greater than reversion frequencies for the markers used. Stability of the recombinants was demonstrated by successive cloning on both complete and minimal medium. Cell contact is apparently required for recombination since both cell types of a mating pair must be present for prototroph formation.