Estimates of population density of all species of fish except the slimy sculpin were made in selected areas of three hard-water streams and three soft-water streams in Pennsylvania. Total standing crops ranged from 46 to 354 pounds per acre. Brown trout (Salmo trutta) stocks ranged from 13 to 137 pounds per acre. Average annual survival of brown trout ranged from 0.189 to 0.554. Recruitment appeared to be fairly stable except for occasional variations believed caused by flood or drought conditions. Growth rate of brown trout was significantly correlated with specific conductance of the water. The data suggest also that a relationship may exist between specific conductivity and standing crop of fish expressed in pounds per acre.