The morphology and the immunohistochemistry of the caudal neurosecretory system ofDicentrarchus labraxwere analysed after the fish acclimatation to fresh water. Two different sized neurosecretory cells have been identified in seawater‐acclimated fish and denoted as anterior and posterior cells. The two types of cells share similar patterns and intensities of immunoreactivity with urotensin I and urotensin II antisera. Posterior neurosecretory cells decreased in size in freshwater‐acclimated specimens, whereas anterior cells did not show changes. Urophyses of seawater‐ and freshwater‐acclimated fish reacted with both antisera. Immunohistochemistry of the urophysis did not provide evidence for a significant difference in the activity of the caudal neurosecretory system between seawater‐ and freshwater‐acclimated fish. However, on a morphological basis it was possible to suggest that the caudal neurosecretory system of the sea bass is involved in the hyper‐osmoregulation.