Pseudomonas alcaligenes NCIMB 9867 (strain P25X) degrades xylenols and cresols via the gentisate path way. P25X expresses two isofunctional gentisate 1,2-dioxy genases (GDO I and GDO II). The expression of both GDOs was not detected when P25X cells were grown at 42 C, even in the presence of gentisate. A total of 19 heat shock proteins (Hsps) belonging to the Hsp100, Hsp90, Hsp70 Hsp60, Hsp45, and small heat shock protein (sHsp) families were identified among the protein spots that were either newly detected or were expressed at levels of at least twofold higher when P25X cells were cultured at 32 or 42 degrees C in the presence and absence of gentisate. Among these, 16 Hsps were commonly expressed at 42 degrees C. Two additional Hsps (H5 and H13) from the Hsp90 and Hsp60 families, respectively, were expressed only when P25X cells were grown at 42 degrees C and in the presence of gentisate. A protein of the sHsp (H 16) family was expressed only in the presence of gentisate at 32 degrees C but not at 42 degrees C. The GroEL chaperonins of the Hsp60 family comprised the largest group of Hsps identified and exhibited high level of expression at 42 C following gentisate I exposure.