Yellow perch, Perca flavescens, from naturally acidic, Wisconsin brownwater lakes survived longer than perch from alkaline lakes when exposed to a lethal pH in the laboratory. Acclimation to a low but sublethal pH for 3 weeks did not remove the difference in acid tolerance, suggesting it is genetically based. Perch from an acidic, low salinity lake (pH 4.6, 13 μequiv. Na/L, 11 μequiv. K/L) had only slightly lower body sodium and potassium levels than perch from a nonacidic, higher salinity lake (pH 7.6,70 μequiv. Na/L, 18 μequiv. K/L). Perch from both lake types were equally susceptible to a lethal high pH, indicating that acid tolerance is not merely the result of an overall hardiness, but reflects specific adaptation to low pH. The exceptional low pH tolerance of yellow perch from Wisconsin brownwater lakes rivals that reported for tropical blackwater fishes.