Pseudobranch function has long interested scientists, but its role has yet to be elucidated. Several studies have suggested that pseudobranchs serve respiratory, osmoregulatory, and sensory functions. This work investigated the immunolocalization of pseudobranch carbonic anhydrase （CA） in the teleost fish species rainbow trout （Oncor- hynchus mykiss） to clarify its physiological function. CA was purified from rainbow trout gills O. mykiss and specific antibodies were raised. Immunoblotting between tissue homogenates of pseudobrench and gill CA antibodies showed specific immunostaining with only one band corresponding to CA in the pseudobranch homogenate. Results of im- munohistochemical technique revealed that CA was distributed within pseudobranch cells and more precisely in the apical parts （anti-vascular） of cells. The basal （vascular） parts of cells, tubular system, blood capillaries, and pillar cells were not immunostained. Immunocytochemistry confirmed these results and showed that some CA enzyme was cytoplasmic and the remainder was linked to membranous structures. The results also showed that the lacunar tissue tayers did not display immunoperoxidase activity. Our results indicated that pseudobranch CA may have a function related to the extracellular medium wherein CA intervenes with the mechanism of stimulation of afferent nerve fibers.
