The mandelate gene cluster for four enzymes has been transduced from Pseudomonas putida strain PRS1 (ATCC 12633) to the camphor-growing, mandelate-deleted strain PUG2 (ATCC 17452). Whereas crosses within strain PRS1 using phage pf16h2 yield principally phage-sensitive transductants, the interstrain crosses of PRS1 and PUG2 yield mostly immune ones, unstable for the mandelate and immune characters. The mandelate cluster is segregated at an approximate frequency of 10 −3 per bacterium per division during the cellular life cycle of the transductants. The immune heterogenotic transductants do not produce active vegetative phages on induction, but upon UV irradiation and superinfection with viable nontransducing phages they produce lysates which transduce at frequencies of 10 −5 to 10 −6 instead of the usual 3 × 10 −8. The release of transducing phages is dependent on the multiplicity of the superinfecting phages and on the dose of UV irradiation. Phage-sensitive transductants of PUG2 also occur, and these also produce high-frequency transducing lysates without producing viable phages. We conclude that the transducing particles consist of different amounts of the phage genome, with or without an immunity-controlling region, and lack some essential phage function. These defective phages have been termed pf dm in analogy to λ dg or Pl dl. In CsCl gradient the infective particles in high-frequency transducing lysates have a characteristic density of 1.519 g/cm 3, whereas three of the pf dm elements tested vary in density from 1.526 to 1.534 g/cm 3, suggesting compositional heterogeneity of the transducing particles. Phage-sensitive transductants of PUG strains can be cured of the defective pf dm particles by ultraviolet irradiation. They contain a satellite DNA band in addition to the bacterial DNA in a CsCl density gradient. These cells overproduce mandelate enzymes, suggesting that pf dm particles replicate autonomously in the phage-sensitive PUG cells.