The authors have characterized a new phage, B86, of Pseudomonas aeruginosa isolated from nature. It is a temperate, uv-inducible, generalized transducing phage. To determine the relatedness of his phage to other characterized P. aeruginosa phages, DNA homology studies were carried out. P. aeruginosa phages have previously been grouped by immunological cross-reactivity. The studies confirm the classification by demonstrating that phages of different class share little or no DNA homology. Based on homology studies as well as cross-immunity to superinfection, B86 is related to other class B phages and is most homologous with phage B39. The virion morphology of these two phages is quite different, however, as are the restriction enzyme digestion patterns of their genomes with several restrictions enzymes.