Within the Caribbean region, Montastrea annularis is considered to be a major contributor to framework construction in both modern and fossil coral reefs. The species forms large colonies over a wide depth range, which in places may extend to as much as 100 m. Growth-rate data for this hermatypic scleractinian are of considerable interest, as, for example, in establishing maximum limits for reef development over long periods of time. Also of interest are the effects of environmental parameters such as water temperature, depth, and light on growth rate. Growth-rate values for Montastrea reported in the literature span a remarkably wide range, with nearly an order of magnitude separating maximum and minimum measurements of annual increment in colony height Long-term monitoring studies, in which the dimensions of a given colony are repeatedly measured, are not only time consuming and costly, but also require that the specimens be handled during the measuring process. Skeletal X-radiography, a new method for determining coral growth, has several important advantages over conventional techniques. From annual density-variation bands in the carbonate skeleton, it is possible to obtain a wealth of quantitative growth data from corals that have lived in their natural reef environment undisturbed by human experimentation. Many years of growth are recorded in a single colony. X-radiographs were taken of 59 specimens of M. annularis and M. cavernosa from five Caribbean sites (Jamaica, Barbados, Key West, Belize, and Panama). Altho gh there is considerable variability in annual growth increment from one colony to another within a population consisting of one species--at one depth and at one reef location--the linear increase in height (measured along the axis of maximum growth) for M. annularis in water less than 3 m deep falls within the range 3 to 12 mm/year. These rates are generally lower than those reported by others, but they are comparable with values determined for another faviid coral, Platygyra, in the Indo-Pacific. The results also indicate that M. cavernosa grows more slowly than M. annularis under similar environmental conditions. Observed growth rates for shallow-water M. annularis are highest at Belize, lowest at Key West, and intermediate at Barbados and amaica. A plot of mean growth rate versus mean annual water temperature at these localities provides a preliminary estimate of the temperature dependence of growth rate: 0.94 mm/year/°C. The result is similar to that for the Indo-Pacific coral Platygyra (0.9 mm/year/°C) which has been studied over the temperature range 23.9 to 29.3°C. It is increasingly evident that, because of the individual variability in growth rates exhibited by corals in their natural habitats, further investigation of the factors that control coral growth will necessitate statistical analysis of large quantities of data. For this purpose, X-ray examination of the skeleton is ideally suited. Seasonal density banding, furthermore, appears to be well preserved in coral fossils, thereby making poss ble the study of coral development in ancient reefs; examples include M. annularis (original aragonite) from elevated Pleistocene reefs of Barbados, and an unidentified faviid coral (recrystallized to calcite) of Miocene age from Saipan.