Abstract The fauna in Pleistocene limestone of Ambergris Cay is homologous with the adjacent Holocene fauna; thus, ready comparison can be made between unaltered and diagenetic samples. Four facies are recognizable in the Pleistocene limestone: (I) a reef-crest facies; (II) a backreef facies; (III) a shelf-lagoon facies composed of outer, middle, and inner shelf zones; and (IV) a mudbank facies. The first two facies and the outer shelf zone correspond closely to equivalent Holocene facies on the Belize shelf; however, unlike any nearby Holocene deposits, the Pleistocene middle shelf zone is oolitic. Facies I and II are biomicrites, III is a biopelmicrite, and IV is a dismicrite. Aragonite persists in corals, mollusks, Halimeda , tunicate spicules, pellets, and ooliths, where not obviously replaced by calcite. Remarkably, magnesian calcite is retained in skeletons of encrusting algae and Foraminifera. Skeletal materials show examples of four general categories of diagenetic alteration: (1) solution- evident in many skeletal and nonskeletal grains; (2) precipitation of calcium carbonate as drusy rims or coarse sparry mosaics of calcite, or as syntaxial overgrowths of the same mineral composition and optical orientation as the grains upon which the overgrowths are based; (3) replacement of aragonitic gastropods by calcite along a jagged “front,” probably accompanied by solution and deposition on a minute scale; ghosts of primary structures typically remain, indicating absence of any major intermediate void stage; replacement of Halimeda and of corals such as Montastrea annularis occurs after occlusion of internal pores by sparry calcite; (4) recrystallization (i.e., alteration of crystals without change in mineralogy)—evident in a few pelecypods, where local patches of shell have been altered to coarse transverse blades of aragonite in which ghosts of primary structures may persist. The limestone was cemented by a combination of interstitial precipitation of drusy and sparry calcite and recrystallization.