The growth rate of the aragonitic skeleton of the Caribbean 'sclerosponge' Ceratoporella nicholsoni has been studied by in situ staining of specimens with calcein in a reef tunnel, 28m depth, near Discovery Bay, Jamaica. Experiments were performed up to five times from 1984 to 1997 on a population of 10 specimens ranging from 10-20cm maximum diameter. In each experiment small skeletal samples were removed from the periphery of sponges, and specimens were left in place for further studies on growth and regeneration. Perpendicular sections, ground to a thickness of about 10μm, were photographed by fluorescence microscopy. Annual skeletal growth rates were calculated from measurements of the linear extension between calcein stained lines along growth axes. Data indicate that although average annual growth rates remained in the same range for different periods (214.6±54.5-233.3±33.0μm yr -1), significant differences occurred from one individual to another within the same period. The annual growth rate of a given individual also varied significantly in time (191.1±30.0-269.9±37.0μm yr -1). A second population of smaller individuals, measured after a single period of one year, revealed a strikingly lower average annual growth rate (124.4±35.0μm yr -1). Regeneration of the skeleton of injured specimens was also characterised by an initial slower growth rate. Nevertheless, after the first year, it was comparable to normal growth, and exceeded it slightly thereafter. This first long term study of Ceratoporella provides important information on the variability in growth rates, with implications on the use of sclerosponges as paleoenvironmental proxies.