Summary Five temperate Agrobacterium phages, two of which are defective, and one presumed temperate, phage PS8, isolated from crown gall tissue, were prepared, purified and compared. The four phages, omega, PS8, PB2A and LV-1, were indistinguishable. They possess a hexagonal head of about 70 nm. with a flexible tail about 200 nm. long. Their DNA had a T m of 92.7° ± 0.1°, a % GC of 56.7 ± 0.2, a mol. wt of 34.1 × 106 and comprised the four normal bases A, T, G and C. No evidence was found for repetitive units in PB2A. These four genome DNA types gave 100% hybridization (limit of error 6%). The electrophoretic protein profiles on SDS polyacrylamide gels are essentially identical: four major bands with similar mol. wt. About half of the phage protein has a mol. wt of 48,000 (perhaps a major head protein), about one-third has a mol. wt of 30,000, one sixth has a mol. wt of 15,500 and 4% has a mol. wt of 69,000. No omega-type prophage DNA was detectable in the cured bacteria v-ic by DNA:DNA hybridization. The latter bacteria were as pathogenic as the prophage-containing parent strain v-i, which throws some doubt on the role of the omega phage group in crown gall induction. The phage-like particle P0362, isolated from the pathogenic Agrobacterium tumefaciens 0362 had a 60 to 70 nm. head and a straight 130 nm. tail. Its DNA was double-stranded with a slightly lower T m of 92.4°, a mol. wt of 25 × 106. The electrophoretic profiles of coat proteins differed from those of the other phages. Its DNA is less than 17% homologous with the omega group. The eventual role of this phage in crown gall induction remains to be established. The phage-like particle P8149, isolated from the non-pathogenic Agrobacterium radiobacter ncib 8149, was quite different. The bipyramidal head was much smaller (40 nm. diameter) and had a very short bud-like tail. The DNA was double-stranded, with a T m of 93.9°, a mol. wt of 10 × 106. The electrophoretic profile of the coat proteins differed from those of the other phages. The DNA does not hybridize with the omega group. As this phage was isolated from a non-pathogenic A. radiobacter, it probably has no function in crown gall. A. radiobacter 8149 is bilysogenic.