A recombination-deficient (Rec-) strain of Caulobacter crescentus has been isolated from a collection of mutants sensitive to ultraviolet irradiation. The Rec- mutant fails to give recombinants following phi Cr30-mediated generalized transduction or following RP4-mediated conjugation. The recombination frequency in the Rec- strain is at least 5000-fold lower than in the wild type strains. The Rec- mutant is indistinguishable from wild type in terms of morphology, growth rate, viability, and phage sensitivities, differing only in properties known to be associated with recA-type mutations in other organisms: recombination frequency, ultraviolet sensitivity, and Weigle reactivation. The map location of the rec-526 allele has not been identified, but rec-526 can be cotransferred with the fla-169 mutation by RP4-mediated conjugation at low frequency. This apparent linkage has been used to move the rec mutation to other strains. The Rec- mutant resembles recA strains of other organisms and provides a healthy strain severely deficient in recombination for use in complementation and cloning studies involving C. crescentus.