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The purification and properties of a lipid-containing bacteriophage, φ 6, are described. The phage contains a lipid envelope which is probably essential for infection. Infectivity of φ 6 was lost in the presence of organic solvents, sodium deoxycholate, and phospholipase A. The fatty acid composition of the phage lipid was similar to that of the Pseudomonas phaseolicola host cells. The phage was composed of about 25% lipid, 13% RNA, and 62% protein. The buoyant density of φ6 was 1.27 g/ml in cesium chloride. The morphology of φ6 was unusual; it had a polyhedral head of about 60 nm surrounded by a membranous, compressible envelope which appeared to assume an elongated configuration upon attachment to pili. The adsorption rate constant was 3.3 x 10-10 ml/min in a semi-synthetic medium and 3.8 x 10-10 ml/min in a nutrient broth-yeast extract medium. The latent period was shorter in the former medium (80-115 min compared with 120-160 min), and the average burst size was larger (250-400 compared with 125-150). The eclipse period coincided with the latent period.