This thesis confirms the hypothesis of Low (1973) that many E.coli K-12 strains contain a prophage (the Rac prophage) located a few minutes clockwise of the txp operon on the genetic map. Restriction endonucleases and 32P-labelled phage RNA probes were used to investigate several E.coli K-12 DNAs and hence construct a physical map of this prophage. Some E.coli K-12 strains have lost the entire prophage by a specific deletion. This is consistent with prophage excision by site-specific deletion. X reverse (x-rev)phages are recombination proficient derivatives of phage 1 in which the phage recombination functions have been replaced by analogous functions (RecE) derived from the host chromosome. The data of this thesis support the origin of x-rev phages by recombination between 1 and the Rac prophage following excision of the Rac genome from the E.coli K12 chromosome. E.coli K12 strains which carry 2L mutations express the Rac exonuclease gene (recE) constitutively. Investigation of the DNAs of several such strains showed them to fall into more than one class. In sbcAS strains a large section of the Rac genome (including a hybrid attachment site and probably the prophage repressor gene) is deleted. Several other sbcA-strains carry multiple (and probably tandemly repeated) copies of the prophage genome. Agar phages are characterised by the replacement of the region of the A genome that contains and the late gene promote, P'R, with host-derived DNA that codes for functions analogous to those deleted. This thesis shows the substitutions in Xgsr phages to be derived from a second (and as yet unlocalised) lambdoid prophage (the gsr Prophage) in E.coli K12.