Temperature-sensitive nitA (rho) mutants of E. coli were isolated; one of them was characterized as an amber mutant. These strains show the Nit phenotype (transcription of phage lambda DNA independent of the N gene) at low temperatures and are inviable at high temperatures. The mutated sites appear to be between cya and metE on the chromosome. Temperature-sensitive nitA bacteria not only permit leftward transcription of the lambda genome at a high rate in the absence of the lambda N protein, but also allow lambda growth at low temperatures. At high temperatures, phages lambda and T4 are incapable of normal development in these cells, while growth of T7 is not affected. The production of thermally unstable rho by the nitA temperature-sensitive mutant suggests that nitA is the structural gene for rho.