We examined relations between mercury concentrations in walleyes Stizostedion vitreum and the characteristics of clear-water Wisconsin lakes, which spanned a broad range of pH values (5.0-8.1) and acid-neutralizing capacities (-9 to 1,017 jieq/L). Total concentrations of mercury in axial muscle tissue of walleyes (total length, 25-56 cm) varied from 0.12 to 1.74 jig/g wet weight.Concentrations were greatest in fish from the eight lakes with pH less than 7.0; concentrations in these fish equaled or exceeded 0.5 /ig/g in 88% of the samples analyzed and 1.0 Mg/g in 44%. In the five lakes with pH of 7.0 and above, concentrations exceeded 0.5 Mg/g in only 1 of 21 walleyes. Multiple regression revealed that lake pH and total length offish accounted for 69% of the variation in mercury concentration in walleyes. Regression models with total length and either waterborne calcium or acid-neutralizing capacity as independent variables accounted for 67% of the variation in concentration. The observed differences in fish mercury concentration between the low-pH and high-pH lakes could not be logically attributed to differences in growth rate or diet among the walleye populations. Moreover, it is improbable that mercury influxes to the low-pH lakes were greater than those to the high-pH lakes, because of the close proximity and spatial interspersion of low-and high-pH lakes. We attributed the observed pH-related trend in mercury concentration in walleyes to variation among lakes in within-lake processes that affected the production and bioavailability of methylmercury.