Four South African scleractinian corals (Favia favus, Favites pentagona, Platygyra daedalea and Gyrosmilia interrupta) and four alcyonacean corals (Lobophytum depressum, Lobophytum venustum, Sinularia dura and Sinularia leptoclados) were experimentally exposed to high sedimentation conditions in the laboratory during a period of six weeks. Experimental sedimentation corresponded to the highest measured sedimentation levels on South African coral reefs, being 200 mg cm-2h-l. Corals were monitored for tissue necroses and bleaching during the course of the experiment and histological sections were prepared after the termination of the experiment. During the experiment, tissue necroses appeared earlier and more frequently in alcyonacea than in scleractinia. Histological sections showed degeneration and necroses of epithelia and mucus-producing cells with accumulation of free mucous material in the epithelia as well as loss of zooxanthellae in all alcyonacea. Local bleaching, due to loss of zooxanthellae, was observed in three alcyonacea (Lobophytum depressum, Sinularia dura, Sinularia leptoclados). Not all parts of the alcyonacean colonies were equally affected by tissue damage and bleaching. In particular, elevated lobes and finger-like projections, which were never covered by sediment for long periods, did not exhibit the same severe damage or bleaching as flat parts of the colonies. Scleractinia did not suffer the same amount of tissue damage as alcyonacea, no bleaching was observed. Partial necroses and degeneration of epithelia as well as changes in mucus producing cells were also observed in scleractinia.