The effect of osmotic shock was investigated mainly in the chloride cells (CCs) and rodlet cells (RCs) of gills, and RCs of intestine and kidney of the European sea bass Dicentrarchus labrax obtained from a farm in the northern Adriatic Sea. During the experiment, fish were abruptly transferred from sea water to a salinity of 15 (15 SW) or to fresh water (FW). Numeric variation and ultrastructural changes of both cell types were evaluated at 24, 48 and 96 h after the transfer to lower salinity levels, using light and transmission electron microscopy (TEM). Exposure to FW produced a significant increase (P < 0·05) in the number of branchial CCs and RCs within 96 and 24 h, respectively. Following osmotic challenge (either transfer to 15 SW or FW), kidney and intestine showed an evident increase in RC numbers. The cellular damage detected by TEM was the same for each sampling time (24, 48 and 96 h), but appeared more severe in fish exposed to FW (higher osmotic shock) than in those exposed to 15 SW. In RCs cytoplasmic vacuolizations, autophagosomes and autophagolysosomes with myelinoid bodies, dissolution and shrinkage of the typical inclusions were documented. Nevertheless, CCs showed vacuolization of endoplasmic reticulum and cytoplasmic dissolution and maintained the apical crypt typical of seawater acclimated fish. Renal tubular cells and intestinal epithelial cells showed similar changes to those reported for CCs and RCs.