Extensive deposits of submarine cements discovered in a submerged Pleistocene cave in the outer barrier-reef platform off Belize, Central America, occur in an unusual setting--on and within countless calcareous projections formed by serpulid worms on the ceiling of the cave. The cements are predominantly dense submicrocrystalline and microcrystalline to porous chalky or submicrosucrosic magnesium calcite (approximately 15 mole % MgCO 3 ), and they contain minor amounts of acicular aragonite, which generally forms a lining inside the aragonitic serpulid tubes. Radiocarbon dates, three-year artificial substrate experiments, and evidence of the ceiling's original relief indicate a low rate of accumulation for the serpulid-cement projections. The characteristic 20-60 mu m peloidal texture associated with magnesium calcite submarine cements is well developed here and there within the projections. These peloids are physiochemical precipitates, having formed during the development of the submarine cement deposits, which accumulated preferentially on substrates that have a slow rate of accumulation and that experience little or no disturbance of their sediment cover. Artificial substrates placed on the ceiling of the cave for three years revealed that the initial stage of precipitation of magnesium calcite from seawater is in the form of a submicrocrystalline magnesium calcite "dust."