Bacteriophage fd has the physical form of a flexible rod. It contains 12.2% DNA of m.w. 1.37 x 106, calculated from the particle weight of the phage (Part II). Base ratios and optical behavior indicate that the DNA is single-stranded. Thymine (34.1%) predominates as is found for other single-stranded DNA phages. Because of its composition and high yield, the phage delivers large amounts of coat protein. Bacteriophage fr contains 30% RNA of m.w. 1.2 x 106. calculated from the molecular weight of the phage (Part II). The RNA, after extraction from the coat protein, forms a more ordered structure. Unpurified preparations of the phages fd and fr contain particles —probably incomplete phages — which are made up of the specific phage coat protein and a reduced proportion of nucleic acid. Convenient methods for the concentration and purification of the phages are described.