Mutations in gene N of lambda prevent killing of the host bacterium after infection. However, derepression of Nsus prophages in nonpermissive (pm(-)) bacteria results in death of the lysogens. When prophages in pm(-)(lambdaCItsA-Nsus) lysogens are derepressed by raising the temperature to 45 C, the cells remain viable as long as they are at 45 C. However, they cannot form colonies at 33 C unless they have been superinfected, at the high temperature, by lambdaCI(+)-Nsus phage which produces repressor at 45 C. A large fraction of these "rescued," heat-inducible lysogens are lysogenized by the superinfecting phage, but lysogenization is not required for rescue. In pm(-)(lambdaCItsA-Nsus) lysogens growing at 45 C, the rate of deoxyribonucleic acid (DNA) synthesis shows a characteristic increase after the temperature is lowered. This increased DNA synthesis, which is correlated with loss of rescue potential, does not occur as long as the cultures are maintained at 45 C.