Preliminary studies have shown that bacteriophages PR3 and PR4, originally isolated on Pseudomonas aeruginosa, resemble the lipid-containing phage PM2 in appearance. Their host range extends intergenerically to species carrying drug-resistance plasmids of the P and N compatibility groups. In this paper, the serological identity of the two isolates is established and it is concluded that they are the same virus, but with some differences in growth characteristics. They contain double-stranded DNA and are probably icosahedra (65 nm) with short (47 nm) noncontractile tails. Their sensitivity to chloroform and low buoyant density in CsCl(1.265 g/ml) indicate that they contain lipid which is probably located in the thickened inner layer of the capsid. A study is made of their adsorption efficiencies to sensitive and resistant bacteria, and it is found that, unlike most sex-specific phages, they adsorb directly to the cell surface and not to sex pili. Their host range is shown to include strains harboring a drug-resistance plasmid of the W compatibility group.