Systematic bottom sampling in the lagoon of Kapingamarangi Atoll [Caroline Islands] indicates that sediments accumulating there are of 6 principal types, each characteristic of and largely restricted to definite depth limits. These types of sediment form facies belts that encircle the deepest parts (240 ft.) of the approximately 6-by-8 mi. lagoon and, on a smaller scale, extend around each of the patch reefs in the lagoon. The facies belts, from the reef inward toward greater depths, are composed of the following materials: (1) medium-grained calcium carbonate sand formed largely of Foraminifera (Amphistegina madagasca riensis and Marginopora vertebralis), (2) fine-grained calcium carbonate sand formed largely of fragments of mollusks, (3) debris from branch corals growing in thickets nearby, (4), accumulations of algal segments of the genus Halimeda, (5) calcium carbonate sand composed largely of a Foraminifera species (Amphistegina lessonii) different from those in shallow water, and (6) calcium carbonate mud. Mechanical analyses indicate that in areas of shallow water, sediments are progressively finer with depth, and a gradation lagoonward occurs from foraminiferal sand into shell sand, the effect of waves and tides. Below the belt of branch corals, at depths greater than about 90 ft., however, sediments of the 3 inner facies belts show virtually no mixing. Where strong currents from the sea enter the lagoon on its southern side, a silt formed from shell and coral fragments has developed a tongue that projects lagoonward for about a mile and disturbs the pattern of facies belts that normally parallel the peripheral reef. The abundance of Foraminifera in most of the sediments at Kapingamarangi Atoll permits a quantitative study of their relation to depth of water and to facies belts, Five large species are important foraminiferal constituents of the sediments. Two occur abundantly in shallow waters up to the crest of the peripheral reef, and 3 occur in waters from 100 to 200 ft. in depth. Other Foraminifera from the atoll are much smaller, occur more sparsely, and are much less important as rock formers; collectively they form in most places less than 1% of the volume of a sample. Foraminiferal content of samples from each of the 6 different facies is distinctive. Relations are apparent between the Foraminifera present, the type of sediment, the depth of water, and the amount of wave action. Quantitative analysis of microfossils in bottom sediments of the lagoon shows that (1) diatoms and sponge spicules are accumulating in greatest numbers in sediments of the relatively deep, quiet water of the center, and are rare in sediments adjacent to the lagoon beaches; (2) microforaminifers are present in all the major sediment facies of the lagoon, but are particularly numerous on bottoms where water is either very shallow or very deep; concentration in deep areas probably is, in part, the result of environmentally induced dwarfing of otherwise larger forms; (3) pollen and spores in the sediments are locally derived, and their extreme scarcity probably reflects the limited number of local source plants; (4) dinoflagellate cysts in the sediments increase in number with depth, and were not found in sediments from water shallower than 150 ft., bathymetric stratification of the adult planktonic forms may be responsible. Possible factors controlling organisms represented in each facies belt are variations in type of bottom, turbulence, and temperature which control the faunas at various depths, and light may determine the lower limit of the alga Halimeda. Other factors of possible significance such as chemical composition, ph, salinity, and density seem to be similar at all depths represented and probably are not important controls. Depth-controlled facies belts, or bathymetric zones, comparable with those at Kapingamarangi, are recognized in some ancient sedimentary deposits, especially in rocks of late Paleozoic age, Certain reefs of several Paleozoic systems are believed to be similar in many respects to the peripheral reef and others to the patch reefs of this atoll.