Properties of some of the bacteriophages of Streptococcus pneumoniae are reviewed. Studies with these phages have yielded several interesting observations and results. (1) A simple transfection system that uses DNA of mature pneumococcal phages was developed; results of studies utilizing this system have determined the intracellular events that follow infection with these bacteriophages. (2) Some pneumococcal phages have shown dependence on the bacterial (host) murein hydrolase for the liberation of phage progeny. (3) Phage Dp-I has been described as the first lipid-containing phage with a gram-positive bacterial host. (4) Phage Cp-l, a recently isolated phage of unique morphology, promises to become a useful phage for studies of the genetics of the pneumococcus.