The National Stream Survey (NSS) conducted by the Environmental Protection Agency in the spring of 1986 sampled 82 stream reaches within the state of Pennsylvania, representing a target population of 9900 stream reaches that are potentially sensitive to acidification. After excluding streams acidified by acid mine drainage, an estimated 9% of the reaches were chronically acidic during spring baseflow at the upstream end while less than 1% of the stream reaches were acidic at their downstream end. Comparable percentages of streams with an acid neutralizing capacity (ANC) between 0 and 50 (mu)eq/L were also identified, with the majority of these streams found in the Appalachian Plateau. Almost all of the chronically acidic streams in the state are small (<20km(2)), upland headwater streams draining watersheds that are more than 95% forested. Population estimates of the percentages of streams with an ANC less than 200 (mu)eq/L agree favorably with estimates by the Pennsylvania Fish Commission for managed and unmanaged stream sections in the state.