The degU (lmo2515) gene encodes a putative response regulator in the food-borne pathogen Listeria monocytogenes. It has 63% amino acid identity to the DegU response regulator of Bacillus subtilis. We have characterized the degU gene product in L. monocytogenes EGD by generation of a deletion mutant. The DeltadegU mutant was found to be non-motile in motility plate assay and no flagellin was detected. The mutant was attenuated in challenge of mice. Northern blot analysis suggested that the degU gene product is a transcriptional activator of the flagellin gene, flaA, at 25 degrees C. However, the degU gene product had no influence on the transcription of prfA encoding the major virulence regulator, PrfA. The results indicate that the putative DegU response regulator is a pleiotropic regulator involved in expression of both motility at low temperature and in vivo virulence in mice.