Data from several biological studies conducted in the Potomac River Basin upstream from Great Falls, Maryland, are presented. Findings of the studies plus other sampling efforts provide an insight into the biological response of the aquatic environment to acid mine drainage and organic pollution in selected stream reaches. The community of macro-organisms was selected as the indicator of the biological conditions in the stream since they serve as the preferred food source for higher aquatic forms and exhibit similar reactions to adverse stream conditions. Tabulated results of quantitative samples are expressed as numbers of organisms per square foot of river bottom and qualitative sampling in numbers of kinds of organisms per sample.