On most coral reefs, the percentage cover of scleractinian corals has declined greatly over the last 30 yr; some species that are more resistant to mortality have been less affected than others. Porites astreoides is one species that has become a more prominent component of coral reef communities throughout the Caribbean. Analyses of coral reefs in shallow water (5 to 6 m depth) at 6 locations spanning a 4100 km arc of the Caribbean were used to evaluate the contribution of P. astreoides to contemporary reefs. Photoquadrats recorded in 2003/2004 were used to estimate the percentage cover and colony density of P. astreoides, and colony size-frequency structure was analyzed to gain insight into demographic processes. At all locations, reefs were characterized by <15% coral cover, but 16 to 72% of this cover was P. astreoides, at mean densities of 1.76 colonies per 0.25 m(2). Most of these colonies (62 %) were <= 50 cm(2) in size, demonstrating that the populations were young and influenced strongly by recruitment. Comparison to historical data collected between 1974 and 1992 suggests that the relative percentage cover of P. astreoides in shallow water reef habitats has increased at a rate of 1.5 % yr(-1), from <20 % in the 1970s to 50 % in 2003/2004. These findings indicate that community structure of Caribbean coral reefs is changing on a decadal time scale to become dominated by 'weedy' corals that form rapidly growing, small colonies that are short lived and quickly replaced.