The goal of this study, supported by the Pennsylvania Department of Conservation and Natural Resources, was to inventory the diatom flora of North-Central Pennsylvania. Although the region has been impacted in the past by logging and mining, development has been minimal compared to most of the state. Some watersheds and water bodies have been restored to a state similar to their natural condition. Many Pennsylvania waters are, however, under a new threat of pollution associated with recent expansion of the Marcellus shale drilling. This study captured composition of diatom communities in summer 2009, before the beginning of massive drilling operations. Some 310 samples were collected from 140 sites in streams, lakes, and wetlands mostly in the protected areas, such as State Forests, Parks and Game Lands. A total of 622 diatom taxa were found, including several species new to science.