Oxygen isotope ratios were measured along the growth axis of branches of the scleractinian coral Acropora formosa collected at 2 and 12 metre depths at Yonge reef (Northern Great Barrier Reef, Australia). Measurements were made between two reference points separated by a distance corresponding to a growth period of six months, from mid-winter to mid-summer. For each of the two reference points, information concerning the environmental parameters controlling the development of the coral colonies was collected in situ. The variability of the isotopic values recorded from coral skeletons grown in identical physical and chemical conditions cannot be ascribed to technical problems, but may rather be induced by calcification mechanisms. In order to define the influence of calcification processes on oxygen isotopic composition, a simple mathematical model is developed, simulating the behaviour of oxygen isotopes. According to Gladfelter's studies (1982, 1983, 1984), we infer that the main factors affecting the isotopic ratio of skeletal aragonite in the scleractinian coral Acropora genus are on the one hand the initial quantity of aragonite deposited at the apical part of the branch and consequently the relative amount of primary and secondary aragonite infilling residual pores during coral growth, and on the other hand, the duration of the secondary aragonite infilling. Comparisons between the measured and the calculated isotopic profiles reveal that differences in calcification processes account for isotopic discrepancies encountered in the different colonies analysed. This study stresses that care must be taken when using the oxygen isotope composition of coral skeleton as a paleoenvironmental proxy.