Corals of the species Acropora squamosa were grown in seawater spiked to varying degrees with calcium chloride. The rate of new growth was measured and it's composition (calcium, strontium, magnesium, and sodium) determined by electron microprobe analysis. Growth rate increased greatly with a moderate (25%) increase in Ca (super 2+) concentration but this increase was less marked at still higher Ca (super 2+) levels. The growth rate declined at concentrations above a 50% increase This suggests that Ca (super 2+) may limit growth at concentrations near seawater but that at higher Ca (super 2+) levels more complex dependencies exist. The Sr (super 2+) /Ca (super 2+) ratio in the new growth proved to reflect that in solution. The same trend was followed to some extent by magnesium but sodium showed no variation at all. This demonstrates clearly that some but by no means all elemental variation in seawater is likely to be reflected in the composition of scleractinian corals.