Restriction maps of genomes of the lytic form and diverse temperate mutants of phage theta of Bacillus licheniformis were constructed. Most temperate mutants produced fragmentation patterns identical to that of the parent lytic form, theta c: in other mutants the only detectable change in the map was the deletion of a Bg/II restriction endonuclease site at 46.5% genome length. In the genomes of two other temperate mutants, theta 1 and theta 2, the central part of the genome was replaced by a piece of DNA of equal length, but with a different distribution of restriction sites; the maps of the two mutants failed to reveal any similarity in the location of restriction sites in the inserted DNA. It seems that any alteration comprising the locus around the coordinate 46.5% of the theta c genome, brings about a transition from the lytic to temperate phenotype, indicating the position of a regulatory gene responsible for positive control of phage replication.