At Mustoe, Virginia, four biofacies of the Lower Devonian Helderberg Group, including a 13-m patch reef, are well exposed. A detailed petrographic analysis indicates that these lime sediments were lithified and altered in two subsurface environments. After growth ceased, the reef was blanketed by nearshore and shoreline facies; deposition at this time was regressive, terminating in regional subaerial exposure. Fresh water then invaded the near-surface sediments, and diagenesis in the meteoric-phreatic zone included rapid neomorphism of some aragonitic fossils, leaching of others, and precipitation of calcite cement with three crystal forms. Overlying limestone facies reflect subsequent subsidence, and the reef was buried progressively deeper. Diagenesis continued in the deep subsurface and included pressure solution, probable smectite-illite conversion, precipitation of ferroan saddle dolomite and coarse calcite cements, and replacement of argillaceous matrix by dolomite. In both subsurface environments, several interrelated diagenetic processes were operating, and the final crystal textures, fabrics, and mineralogies were a consequence of this interplay.