The environmental interpretation of sedimentary rocks is one of the principal objectives of the sedimentologist and stratigrapher. In the past, such interpretations have been based primarily on certain diagnostic fossils and the gross petrography, criteria which have obvious limitations. In recent studies, Degens et. al. have shown that geochemical criteria can be successfully used as environmental indicators for most sedimentary rock types. The purpose of this investigation is to study the application of stable isotope abundance measurement as possible environmental indicators. C and O isotope analyses of the carbonate fraction were made on samples of limestone, sandstone, shales, and some fossil shells. The majority of the samples were collected from the Allegheny series of Clearfield County [Pennsylvania] and have been classified into fresh-water and marine groups in previous investigations. The remainder of the samples were collected from different formations and localities and have been classified into marine and fresh-water groups on the basis of fossils.