Coral reefs are known to flourish typically in clear oligotrophic waters but, contrary to this dogma, many reefs occur under marginal conditions such as high turbidity and/or high sedimentation environments. The question we pose is: how are reefs adapted to such marginal environments? As a model system we studied the reefs of the turbid Bay of Banten, NW Java, Indonesia. Reef surveys (transects) along a string of islands showed changes in community characteristics (inshoreoffshore): Coral colony partial mortality decreased, coral cover and species richness increased. Asexual recruitment was dominant on all reefs, but sexual recruitment was still occurring (~10%). High turbidity (k' = 0.17-1.26) and sedimentation (2.5-63 mg cm-2day-1) did not correlate with reef development. Resuspension of bottom sediments, possibly preventing negative effects of sedimentation, paralleled patterns in reef development. Variation also occurred at the level of the coral organism. Chlorophyll a levels were high in coral colonies (Porites massive) in turbid conditions compared to less turbid conditions (depth < 4m, 36 versus 15 ?g cm-2). Most of the changes in community structure are probably the result of phenotypic adaptation at the organismic level. There are also indications of adaptation sensu stricto, representing genetically based variation. We found RNA/DNA ratios in coral tissue, which are presumed to reflect tissue growth characteristics, to be consistently higher in corals living in turbid environments. However, under the most extreme turbidity conditions RNA/DNA ratios were not higher. Apparently, corals are able to adapt to some degree to higher turbidity levels.