The skeletal extension and corallite shape of individuals of the Caribbean and Atlantic reef-building coral Montastrea annularis (Ellis and Solander) were measured after more than six weeks' continuous flow-through exposure in laboratory aquaria to treatments of 0, 1, 10 and 100-ppm (μl 1-1) drilling mud. Linear increase of the skeleton (extension rate) and fossa length were significantly depressed in the 100-ppm treatment. Chronic exposure to 100-ppm drilling mud impairs coral skeletal growth rate and possibly interferes with sediment rejection capability by lowering calical relief.