HSP90 proteins are chaperones that play a pivotal role in controlling multiple regulatory pathways such as stress defense, hormone signalling, cell cycle control, cell proliferation and differentiation, and apoptosis. In this study, two cDNAs encoding for cytosolic HSP90, referred to as HSP90AA and HSP90AB, have been sequenced. Main features and sequence identities with other fish and mammals are described. Phylogenetic analysis grouped both genes into two separate clusters with their fish and mammalian counterparts. Expression profiles during larval development and in juvenile tissues were analyzed using a real-time PCR approach. In juvenile fish, HSP90AB was constitutively expressed with lower transcript levels in skeletal muscle. In contrast, HSP90AA was mainly expressed in heart, skeletal muscle and skin. During metamorphosis, HSP90AB mRNA levels did not change whereas HSP90AA transcripts decreased significantly at the beginning of metamorphosis with the lowest mRNA levels at the metamorphosis climax. Due to the role of thyroid hormones (THs) on sole metamorphosis, the transcriptional regulation of HSP90 genes by THs was evaluated. Larvae exposed to the goitrogen thiourea (TU) exhibited higher HSP90AA mRNA levels than untreated control. Moreover, adding exogenous T4 hormone to TU-treated larvae restored the steady-state levels with respect to the untreated control. Unlike HSP90AA, the transcript levels of HSP90AB did not vary under any treatments. The response of both HSP90 genes to thermal stress in post-metamorphic individuals was also studied. A heat shock treatment (+7.9 degrees C for 1 h) rapidly activated HSP90AA (but not HSP90AB) transcription, reaching a peak after 30 min and declining expression levels progressively in the following 24 h. No significant changes in HSP90AA or HSP90AB transcript levels after a cold shock (-10 degrees C for 1 h) were observed. Overall, these results demonstrate that HSP90AA transcription is down-regulated by THs and up-regulated after a heat shock in Senegalese sole.