Carbonate sediment samples collected in various reef and lagoonal environments at Eniwetok Atoll, Marshall Islands, were subdivided into six size fractions and analyzed for mineralogic, isotopic, and faunal composition. The goals of this investigation are an understanding of C 13 and O 18 distribution in sediments associated with coral reefs and elucidation of the factors responsible for variations in the stable isotope ratios of carbon and oxygen. C 13 /C 12 in carbonate sediments at Eniwetok is dependent upon the relative proportions of biogenic carbonate minerals, with different C 13 concentrations, derived from different types of marine organisms. Some of the important carbonate contributors fractionate carbon and oxygen isotopes. As ecological restrictions rigidly control the distribution of calcareous organisms in a reef complex, a correlation between isotopic composition of the sediment and type of environment is induced by causal relationships between isotopic composition and type of organism and between type of organism and environment. The C 13 to C 12 ratio of sediments forming in reef areas is considerably lower than that of lagoonal carbonates, reflecting differences in the relative abundance of C 13 -enriched aragonite from Halimeda and C 13 -deficient aragonite from corals in these two major environments. Where the ratio of these two carbonates changes abruptly, for example in the vicinity of patch reefs and coral pinnacles in the lagoon, corresponding changes, equally abrupt, in C 13 /C 12 are found. For oxygen isotope ratios, a small temperature effect is superimposed upon the genetic factors, which, as for carbon, are the dominant factors.