When ultraviolet-irradiated bacteriophage T4 is assayed at plating temperatures ranging from 20° to 40°, its survival increases at the higher temperatures. This "thermal rescue" requires an intact WXY system but not the denV pyrimidine dimer excision system. Mutation rates decrease with increasing temperature, indicating that some lesions processed in a mutagenic manner at lower temperatures are accurately repaired or circumvented at high temperatures. When both the cold sensitivity of UV survival in the wild type and the temperature sensitivity of newly isolated ts mutants of uvsX and uvsY were used, expression of the WXY system was monitored in temperature shift UV survival experiments and was found to be biphasic: the uvsX and uvsY functions increase UV survival in two increments, one at an early and another at a late stage of infection. The uvsW function, however, increases UV survival only early in infection.