Brook trout Salvelinus fontinalis, mottled sculpins Cottus bairdi, and slimy sculpins Cottus cognatus occur in many Pennsylvania streams that have depressed pH and elevated aluminum concentrations during episodes of high stream discharge (acidic episodes). The authors performed 20-d in situ cage exposures with these species to determine their relative sensitivities to field conditions. They also exposed fish in the laboratory to synthetic soft water, without added Al, to elevate possible effects of Al on sodium flux rates and pH toxicity. Exposures were in five streams; two with high pH (<5.60) and low Al concentrations (<80 micrograms/liter) and three with low pH and high Al levels. Exposures were during two low-discharge fall periods, when pH tends to be seasonally higher and Al concentrations lower, and two relatively high-discharge spring seasons, when lower pH and higher Al concentrations are typical. Few fish died (generally <10%) in the two streams that had higher pH and lower Al concentrations, whereas mortalities typically exceeded 20% and were as high as 100% during spring exposures in the streams with lower pH and elevated Al concentrations.