SUMMARY1In the lysogenic bacterium the bacterial virus exists as prophage. It is composed of desoxyribosenucleic acid (DNA) and occupies a defined site on the bacterial chromosome.2Normally the prophage divides in harmony with the nucleus of the bacterium, so that lysogenesis is hereditary.3Occasionally the prophage reverts from symbiosis to virulence, and multiplies at the expense of its host, producing a brood of free phage particles. This is believed to occur when the prophage becomes detached from the chromosome.4Free phage particles may be temperate (capable of reproducing lysogenesis in a sensitive bacterium) or virulent (unable to produce lysogenesis and multiplying only at the expense of the doomed host). Temperate particles greatly outnumber virulent particles, which are in fact mutants. Reasons are given for believing that all virulent phages (such as the T series of coliphages) originate in this way.5Free phage particles have a characteristic tadpole‐like structure of varying proportions. The tail and the covering of the head are composed of protein, which has specific antigenic characters. The contents of the head consist of DNA.6The protein envelope and tail of the particle are chiefly mechanical in function. The tail adheres to the host bacterium and breaches its covering membrane. The contents of the head are then injected into the bacterium. They constitute the infective element.7A temperate phage may produce lysogenesis or may multiply in the same way as a virulent particle. Various factors facilitate or interfere with the establishment of lysogenesis. There is evidence to suggest that some temperate particles are specially endowed to produce lysogenesis, while others can undergo only productive development at the expense of the host.8Virulent particles multiply only by productive development. The infecting virus displaces the genetic apparatus of the bacterium and directs the bacterial enzymes to its own ends. The nucleic acid of the bacterium, augmented if necessary by newly synthesized material from the culture medium, is broken down to nucleotide level or lower, and resynthesized into viral DNA. Protein is synthesized separately. Ultimately both are assembled into mature particles.9Lysogenic bacteria are immune to the phage they produce, and to allied types. Occasional virulent mutants are exceptions to this rule.10In certain cases temperate phage particles transduce genetic characters from one bacterium to another.11Bacterial species (the classical example beingSalmonella typhi) can be broken down into numerous ‘phage‐types’ by tests which exploit the hereditary immunity conferred by the various symbiotic phages. Alternatively, different races of a bacterial species can be recognized by isolating and identifying the symbiotic phage which the