The life history of two populations of Etheostoma olmstedi Storer in the Patuxent River, Maryland, one in cold reservoir tailwater and the other in sewage-polluted water, was studied to determine the effects on this species of man-made modifications in the stream. The spawning season was in May and June with temperatures approximately 12.5–14.5 C in the cold tailwater and 16.0–18.5 C in the sewage-polluted water. In the sewage-polluted water, the fish grew faster in early summer of the first year than those in cold tailwater, but later on growth in length slowed down, even though the condition factor continued to increase. Fecundity in the sewage-polluted water population was larger than that in the cold tailwater population because of larger body weight. In the sewage-polluted water, there were fewer males with a lower condition factor than females and had a higher percentage of fish in the II-year class and a lower percentage in the I-year class than the cold tailwater population. Males did not grow as fast as females in the cold tailwater.