DNA microarrays were employed to compare gene expression in a thermotolerant, nalidixic acid-resistant mutant of Escherichia coli with that of the parental strain. When grown at 37 degrees C, up-regulated genes in the mutant included those coding for multiple antibiotic resistance proteins and enzymes for the degradation of small molecules, whereas among the down-regulated genes were those coding for fimbrial, flagellar, and outer membrane proteins as well as sigma38. When the mutant grown at 42 degrees C was compared to the mutant grown at 37 degrees C, enhanced expression of several genes coding for flagellar proteins was detected. Reverse transcriptase-polymerase chain reaction analysis of selected genes confirmed results obtained with microarrays.