The impacts of a large airport on the water quality and aquatic life of a small (94.8 km2drainage area) western Pennsylvania stream were examined. The principal adverse effects of airport runoff were related to runway deicing operations. Microbial metabolism of glycols and urea utilized as deicing reagents exerted a strong biochemical oxygen demand in receiving waters. Similarly, the breakdown of urea led to elevated concentrations of ammonia. The use of alkaline mill slag as a fill material in the construction of the runways was probably responsible for some elevated alkalinity and pH values from runway leachates. The organic waste load stimulated the growth of dense biological slimes on streambeds. Invertebrate communities in waters influenced by airport runoff were severely stressed and dominated by pollution tolerant Chironomidae and Oligochaeta. The fishery of the watershed was also impaired, although transient fishes from the nearby Ohio River apparently reinvaded the stream at certain times of the year.