Corynebacteriophage 782, a phage highly related to the beta family of corynebacteriophages but lacking a tox allele, was isolated from a nontoxinogenic clinical isolate of Corynebacterium diphtheriae. Phage 782 exhibits beta immunity but has a wider host range than beta, forming plaques on strains of C. ulcerans and C. pseudotuberculosis as well as on C. diphtheriae. Phage 782 and beta differed in their DNA mass and in their restriction endonuclease digest patterns, but were similar in possessing cos (cohesive) and attP (phage attachment) sites. Moreover, all the BamHI fragments of 782 and beta except one hybridized with a DNA probe of the other. The exception in both cases was the attP-containing fragment, which in beta also carries the tox gene. Recombinants between phage 782 and pi phage, a tox+ beta-related phage, were isolated which contained ca. 70% of phage 782 DNA but carried the attP-tox-bearing fragment of pi and were thus now converting phages. The recombinants had lost the wide-host-range phenotype of 782 and had the narrower host range of pi. The significance of the tox-less, beta-related phages to the natural history of diphtheria is discussed.