A new virulent bacteriophage, designated RZ1, was isolated from a local pond on the facultative phototrophic bacterium Rhodobacter sphaeroides ZZ101. Electron microscopic studies revealed that, in general morphology, phage RZ1 resembles the lambda-bacteriophage of Escherichia coli. The host range of phage RZ1 is limited to some strains of R. sphaeroides. The phage genome consists of double-stranded DNA of about 44 kb lacking cohesive ends and seems to present terminal redundancy and cyclic permutation. RZ1 phage may carry out a lytic cycle only in recombination-defective mutants of R. sphaeroides. Nevertheless, a derivative of the RZ1 phage, termed RW1, able to grow in recombination-proficient strains of R. sphaeroides, has also been obtained. In vitro restriction analysis of both RZ1 and RW1 phages shows the presence of a rearrangement in their DNA. Generalized transduction of Str(r) and Rif(r) chromosomal markers has not been detected with either RZ1 or RW1 phages.