This volume is a detailed, scientific survey of all fishes in the state of Pennsylvania including their distribution throughout the several watersheds of the state. It will contain several short introductory chapters on geology, glacial history, climate, and other ecological factors pertinent to the understanding of the present distribution of fish species in the commonwealth. Also included is a brief treatment of the fish fossils, the evolutionary origins of present species, and their probable routes of dispersal between major drainage systems. Sufficient historical records exist to compare with recent collections to indicate changes in fish populations over the past 150 years, perhaps correlated with severe environmental disturbances by man. The major portion of the volume will consist of illustrated keys to the identification of about 190 species of fishes arranged in phylogenetic order. Spot distribution maps for each species, prepared from nearly 1,500 fish collections in Pennsylvania, are included with black and white photographs and numerous line drawings of important morphological characters.