ABSTRACT: Competition between corals and algae is an important process on coral reefs, especially during reef degradation, when abundant corals are often overgrown by benthic macroalgae. Despite the widespread assumption that macroalgae are able toout-compete corals for space, there have been very few experimental studies testing the nature of this interaction. This study compared the effects of a filamentous red alga, Anotrichium tenue, with those of mixed-species, filamentous algal turfson massive Porites spp. corals on inshore reefs of the central Great Barrier Reef, Australia. We compared mortality of coral tissue in plots with A. tenue naturally present on live coral tissue, plots in which A. tenue was naturallypresent but experimentally removed, and plots where mixed algal turfs were naturally present but A. tenue was not. The results indicate that A. tenue killed coral tissue by active overgrowth. Removing the alga removed the effect. Incontrast, general, mixed-species algal turfs did not cause any mortality of coral tissue. We suggest that 2 particular traits of A. tenue may facilitate its effects on the corals. First, unlike most filamentous turf species present, it was able toovergrow live coral tissue, perhaps due to allelochemical effects. Second, individual algal filaments trap relatively large amounts of mucus from the corals and of sediment, apparently increasing the damage to underlying coral tissue. Surveys indicatedthat A. tenue primarily affected massive Porites spp., that overgrowth effects were not site-specific, but that occurrence of infected corals was not widespread. In particular, distribution patterns were not consistent with an effect ofterrestrial runoff. This study provides evidence of an exceptionally lethal effect on corals by a single species of filamentous alga, and emphasizes the species-specific nature of coral-algal competitive outcomes, even within a functional group.