We investigated mRNA expression patterns in the laboratory-reared diatom Chaetoceros compressum under temperature stress conditions. The diatom incubated at 30°C for 15min, showing 43% growth rate in a subsequent culture at 20°C for 2 days, were analyzed by mRNA arbitrarily primed reverse transcription-polymerase chain reaction (RAP RT-PCR) in comparison with control cells incubated at 20°C throughout. The heat stress at 30°C induced expression changes in 7 cDNAs, one of which showed 37% identity to a homologous region of the Bacteroides fragilis glutamine synthetase gene. This stress on the diatom resulted in 2.5-fold increase in its mRNA levels. Our results demonstrated that the gene (s) related to this cDNA is expressed in response to heat stress for the diatom, suggesting its possible applications to monitoring marine environments.