Coring of the Mariana Limestone on the Orote Peninsula of western Guam has created an opportunity to examine in detail the porosity evolution of a Pleistocene reef complex. Four cores acquired for study are composed predominantly of skeletal packstones, grainstones, and rudstones, with only minor bound-stone development. Deposition occurred within the reef-flat to proximal back-reef sand apron facies. Considerable primary interparticle and intraparticle porosity has been occluded by multiple-generations of submarine cementation and internal sediment infilling within the marine environment. In some instances, multigenera-tion, radiaxial to radial fibrous, Mg-calcite, isopachous rim cements have completely occluded all primary interparticle porosity...