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The structure of the gill epithelium of adult whitefish (Coregonus lavaretus) was studied in the laboratory at pH 4.75 and pH 5.75 with and without added aluminium (Altot 150 μg 1−1). The gills of whitefish and perch (Perca fluviatilis) from acidic and circumneutral lakes were also studied. The increase in the mean arithmetic thickness of the lamellar epithelium of whitefish gills was not significant at pH 4.75 + Al, when compared with the control fish. The morphometric diffusing capacity decreased significantly, however. It was also significantly smaller in the gills of the whitefish in the acidic lake than in the circumneutral lake. Perch gills were not hypertrophic in any lake. The water pH or Al concentration had no consistent effect on the number of mucous cells in the gill epithelium of whitefish in the laboratory exposure, but mucous cells in the acidic lake whitefish were hypertrophied. No mucous cells were present in the lamellar region of the perch caught in lakes with a low Al concentration. Al was found inside the gill epithelium of whitefish both at pH 4.75 and pH 5.25 when Al (150 μg 1−1) was added to the water in the laboratory exposure. The gills of the whitefish accumulated Al in an acidic lake, also. Al was found inside the perch gill epithelium when the ambient water pH was circa 5, even when the total Al concentration of the water was low.