Phage beta 197tox-, which codes for CRM197, a nontoxic protein immunochemically indistinguishable from diphtheria toxin, was UV induced from a culture of the C7(beta 197)tox- strain. A total of 191 C7(beta 197)tox- lysogens were isolated and selected according to the halo produced on TYE agar containing antidiphtheria toxin serum and were further characterized by Southern blots of their chromosomal DNA. Most of the isolates turned out to be monolysogens, but some tandem and nontandem double lysogens were also found. The nontandem double lysogens were stable and capable of giving high yields of CRM197, up to threefold higher than monolysogens. They are, therefore, suitable for large-scale industrial production.