Salmonella typhimurium thermotolerant mutants dependent on the presence of nalidixic acid for growth at 48-degrees-C were isolated and designated nalidixic acid-dependent, thermotolerant mutants, nal(d)ttl. Genetic mapping revealed that nal(d)ttl alleles map within the gyrA gene. When S. typhimurium strain Q was plated in the dark on nutrient agar containing nalidixic acid (20 mug/ml) as a photosensitizer and briefly exposed to white light or near VU light prior to incubation at 42-degrees-C, nalidixic acid-resistant mutants arose in about 16 h at frequencies of 5 x 10(-8) for white light and 1 x 10(-6) for near UV light. About 10% of these nalidixic acid-resistant mutants derived from photodynamic mutagenesis exhibited the thermotolerant characteristic.