The colonization, taxonomic succession and marginal growth and accretion rates of crustose corallines on artificial substrates in algal ridge and reef environments on St Croix, U.S. Virgin Islands were examined . Very thin Leptoporolithon and Tenarea species are the initial colonizers of glass and plastic plates placed in these environments. In strong light conditions, the colonizers are followed by Neogoniolithon and Porolithon species. In areas of high wave energy, where the activities of grazing organisms are greatly reduced, the climax species Lithophyllum congestum and Porolithon pachydermum are capable of building intertidal algal ridges. The dominant coralline crusts showed marginal crustal extension rates of 0.9–2.3 mm/month and accretion rates of about 1–5.2 mm/year. Mean rates are about an order of magnitude greater than those previously measured in subarctic waters. The accretion rates are highly dependent upon the grazing activities of animals, especially parrot fish. Maximum rates in...