The Stream Conditions of Chester County Biological Monitoring Network (Network) was established by the U.S. Geological Survey and the Chester County Water Resources Authority in 1969. Chester County encompasses 760 square miles in southeastern Pennsylvania and has a rapidly expand-ing population. Land-use change has occurred in response to this continual growth, as open space, agricultural lands, and wooded lands have been converted to residential and com-mercial lands. In 1998, the Network was modified to include 18 fixed-location sites and 9 flexible-location sites. Sites were sampled annually in the fall (October-November) during base-flow conditions for water chemistry, instream habitat, and ben-thic macroinvertebrates. A new set of 9 flexible-location sites was selected each year. From 1998 to 2009, 213 samples were collected from the 18 fixed-location sites and 107 samples were collected from the 84 flexible-location sites. Eighteen flexible-location sites were sampled more than once over the 12-year period; 66 sites were sampled only once.