Study of many of the interesting properties of Klebsiella aerogenes is limited by the lack of a well-characterized genetic system for this organism. Our investigations of the evolution of the enzyme ribitol dehydrogenase (EC 1.1.1.56) in K. aerogenes would be greatly facilitated by the availability of such a system, and we here report two approaches to developing one. We have isolated mutants sensitive to the coliphage P1, which will efficiently tranduce genetic markers between such sensitive strains and which will thus make detailed mapping studies possible. Derivatives of K. aerogenes lysogenic for P1 can be readily isolated by using the specialized transducing particle P1CMclr100. Bacteria lysogenic for this phage are chloramphenicol resistant and temperature sensitive. Phage particles produced by temperature induction of such lysogens can be used to transfer K. aerogenes genes to the natural host of P1 phage. Escherichia coli. We have used this method to prepare derivatives of E. coli K-12 carrying the K. aerogenes genes conferring the ability to metabolize the pentitols ribitol and D-arabitol. We have shown that these E. coli-K. aerogenes hybrids synthesize a ribitol dehydrogenase with the properties of the K. aerogenes enzyme and have mapped the position of the transferred gene on the E. coli chromosome. The ramifications of this methodology are discussed.