A mutation that causes a temperature-sensitive RecA(-) phenotype was identified in a derivative of a PolA(-) strain that failed to grow at high temperature. The mutant allele (recA200) was shown to be linked to cysC, conferred a sharply temperature-sensitive, ultraviolet-sensitive Rec(-) phenotype in the range 35 to 42 C, and in crosses failed to show complementation at 42 C with Hfr's that transferred recA(-). Double mutants that carried both recA200 and polA were examined for ability to grow and synthesize DNA at restrictive temperatures.