SUMMARY : The staphylococcal bacteriophage 3 A consists of a flat, oblong head, c. 600A. by c. lOOOA., and a long tail, c. 120A. by c. 2900A. Fixation with formalin destroys the integrity of the head. Cells infected With the phage release it at the end of the latent period by a process resembling the bursting of a bag. After the phage is released, the empty cell membranes persist for a short time before being dissolved in the medium. Correlation of these findings with turbidity readings and Gram staining of the infected cultures indicate that the turbidity falls only when the shells of the cocci are dissolving. Electron microscopy has been used extensively to study the morphology of the coli T series of bacteriophages and the processes of phage adsorption and cellular lysis with these particular host-virus systems. There have been fewer studies of staphylococcal bacteriophages. Wyckoff (1949) published one photograph of the staphylococcus phage K. Included in the paper of Smiles, Welch & Elford (1948) were unshadowed photographs of cells lysed by phage K in which the phage particles are arranged radially around the remnants of the cells. Hotchin, Dawson & Elford (1952) photographed phage K adsorbed tail first to the cell membranes of staphylococci that had been previously disrupted ultrasonically. This paper is concerned with the staphylococcal phage 3 A, originally isolated by Wilson & Atkinson (1945), the growth characteristics of which have been described elsewhere (Rountree, 1952). The object of the investigation was to study the morphology of the phage and the release of the phage from infected cells. EXPERIMENTAL Preparatiolz of the material The material used consisted of: