The orientation of the gene for diphtheria toxin, tox, in the prophage of converting corynebacteriophage beta has been determined. The orientation of tox in prophage and that reported simultaneously by Holmes (1976) for vegetative phage are compatible with the hypothesis that beta phage is inserted into the chromosome of its bacterial host by means of a mechanism similar to that described for lambda phage, and that the phage attachment site lies between the tox and imm genes. The position of three tox mutations that are phenotypically CRM- has also been determined. Relative to the tox-45 mutation, they are located more proximally to the end of the tox structural gene that corresponds to the amino terminal of diphtheria toxin.