Most research on the adaptation of thermophiles is focused on their adaptation to heat stress; only a few studies are focused on their cold adaptation. In this report, the thermophilic bacterium Rhodothermus sp. XMH10 was examined to gain a better understanding of gene expression in response to low temperature. Random arbitrarily primed polymerase chain reaction (RAP-PCR) was used to isolate and identify differentially expressed genes of bacteria grown at 45 degrees C (lowest) compared to those at 75 degrees C (optimal). Fifty-three differential cDNA fragments in total were isolated. Among them, 35 different cDNAs were analyzed by Northern blot, and 17 were confirmed to be differentially expressed at the transcriptional levels. These genes reflected a profile of differential expression and were involved in many physiological processes such as metabolism, cell membrane alterations, and regulatory adaptive response; most of them have never been previously reported. This study provides some new information on the adaptation of thermophilic bacteria to environmental temperature stress.