In a group of 138 northern Wisconsin lakes (pH 4.0–9.2), we observed differences in the lowest pH at which 31 fish species occurred. The central mudminnow (Umbra limi), yellow perch (Perca flavescens), and various centrarchids occurred below pH 5.0, whereas many cyprinids and darters were not found below pH 6.2. Rankings of species occurrence with minimum pH in these naturally acidic lakes were similar to data from Canadian lakes affected by acid precipitation, and rankings based on survival during laboratory exposure to low pH. Wisconsin oligotrophic lakes that are susceptible to acid precipitation typically lack many fish species sensitive to low pH, even when lake pH is near 7.0. Species interactions and biogeographic factors, not recent cultural acidification, are probably responsible for species absences in these lakes. Knowledge of the pH ranges naturally tolerated by fishes and the structure of fish assemblages in lakes sensitive to acid precipitation will aid in detecting changes caused by cultural acidification.