Adult brook trout (Salvelinus fontinalis) were exposed for 147 d to three different combinations of acid and Al in soft water (2 mg Ca/L). Samples of gill tissue from each of the three exposure conditions (pH 6.64 + 0 micrograms Al/L, pH 4.91 + 4.3 micrograms Al/L, and pH 4.45 + 393 micrograms Al/L) were examined by light microscopy using high-resolution techniques and morphometric analysis. As compared with fish in control conditions (pH 6.64 + 0 micrograms Al/L), exposure to pH 4.91 without Al significantly increased volume density of lamellar chloride cells. Low pH and elevated Al (pH 4.45 + 393 micrograms Al/L resulted in increased diffusion distance, while blood cell infiltration of the lymphatic space of the gill tissue (an indicator of tissue damage), and a dramatic increase in lamellar dense cells. These results are consistent with the physiological data that show ionoregulatory stress and decreased respiratory efficiency as typical responses of brook trout exposed to low pH and combinations of low pH and elevated Al.