The importance of marine versus terrestrial foods in prehistoric Pacific and New Zealand diets, and the adaptation of the Polynesian diet to new enviroments, is examined through the analysis of the ratios in human bone of the stable isotopes of carbon, nitrogen and sulphur. In particular, this study seeks to obtain quantitative information which could provide answers to five main questions, relating to the subsistence focus of the early Lapita colonists in the Pacific, the significance of suger cane in the diets of early Pacific populations, the proportions of reef versus open ocean and terrestrial versus marine foods in these diets, and the identification of populations with pronounced marine or pronounced terrestrial diets. One hundred and nineteen samples of human bone from 13 sites throughtout the Pacific and New Zealand were processed. Nitrogen values were obtained directly from bone powder, while carbon values were determined from collagen produced by digesting bone powder in phosphoric acid. Sulphur evaluations were determined from a BaSo⁴ precipitate, produced after combustion of the collagen samples in a Parr bomb. Interpretation of results is approached from a comparative point of view, which enables the proportions of marine and terrestrial foods in the diets of each study group to be assessed in relation to the diets of all the other groups. Additional information on the composition of the diets is gained by comparing the stable isotope values obtained in this study with published values of other human populations, and of marine and terrrestrial plants and animals.The potential of stable isotope analysis to identify the composition of prehistoric New Zealand and Pacific diets is confirmed. A unique marine adaptation is revealed from the analysis of the Chatham Islands Moriori who appear to have focused almost exclusively on marine resources. In contrast, a highly terrestrial diet is suggested for groups from Nebira in Papua New Guinea and Lake Rotoiti in New Zealand.