"Petrographic fossil diversity" is defined as the total number of all dissimilar fossil types seen in thin section. Although such fossils are identified to several different and usually broad taxonomic levels, petrographic fossil diversity parallels simple species diversity of the paleontologist. Thus, in a petrologic investigation of a sedimentary formation, a non-paleontologist can tabulate and plot a first approximation to total and mean diversity for each microfacies, a diversity gradient between paleoenvironments, and a diversity log for the stratigraphic section. In the absence of detailed paleontologic work, fossil diversity may still be considered, and any ecologic interpretations based on fossil diversity may still be made. In summary, petrographic fossil diversity is a quantitative measure to aid in reconstructing ancient sedimentary environments.