The chapter discusses on the various small bacteriophages. These phages are quite heterogeneous; some species contain DNA and others contain ribonucleic acid (RNA); some particles have cubic symmetry and others are made of a helical nucleoprotein structure. The chemical nature of the nucleic acid and the system of symmetry in the virus particle are reasonable criteria for classification, accordingly, the small bacteriophage can be divided into three groups: (1) isometric, DNA phages; (2) Rod-shaped DNA phages; and (3) isometric RNA phages. The chapter discusses various isometric DNA phages, such as the viral strains, the φx-174 particle, the DNA, φX coat protein, radiobiology and chemical mutagenesis, genetic recombination, the reproductive cycle of φX. Interest in small bacteriophage has arisen predominantly because these viruses contain single-stranded polynucleotide and because the reproduction of single strands allows the study of nucleic acid function under aspects different from those examined with bihelical DNA-containing objects. The chapter also describes various rod-shaped DNA phages, such as the viral strains, the virus particles, the DNA, the coat protein of fd , initiation of infection, and phage liberation. It is tempting to speculate that the two processes of irregular association have a common cause, and that deductions made for the RNA virus also hold for the DNA virus— that is, self-assembly of the viral nucleic acid and capsidal protein with small bacteriophage must be complemented by an unknown function of the phage. The chapter discusses various isometric RNA phages, such as the viral strains, the virus particles, the RNA, the coat protein, radiobiology, and the reproductive cycle.