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played an important role in sample collection. Linda Zarr contributed her expertise to the data analysis. Scott Hoffman provided valuable graphics support. Kim Otto (retired) provided editorial support. USGS reviewer Kevin Breen provided a particularly insightful review of an early draft of the manuscript. Other USGS reviewers, Lisa Senior, Pat Philips, and Doug Chambers offered valuable suggestions for improving the manuscript. The interest and support of Pat Bowling of the PADEP is greatly appreciated. The authors also wish to extend thanks to the landowners and the wastewatertreatment-plant operators who graciously allowed access to the wells and streams. To all who assisted in any way, the authors convey sincere thanks.

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Physical, chemical, and biological data were collected at 51 sampling sites in Chester County, Pa., from 1970 through 1997 as part of the Stream Conditions of Chester County Program. This report presents data collected from 43 sites from 1995 through 1997 that constitute a continuation of the program. Physical data include water temperature, instantaneous stream discharge, pH, alkalinity, specific conductance, and dissolved oxygen. Chemical data collected include laboratory determinations of nutrients and major ions in whole water samples and selected metals, pesticides, gross polychlorinated biphenyls (PCB's), gross polychlorinated napthalenes (PCN's), and total carbon in stream-sediment samples. The biological data include benthic-macroinvertebrate populations. The data are presented without interpretation. Chester County is undergoing urbanization as agricultural land is converted to residential developments, commercial areas, and industrial and corporate parks. The major goal of the Stream Conditions of Chester County Program is to further the understanding of stream changes in response to urbanization.