The skeletons of living specimens of the scleractinian coral Porites lobata have been found to contain up to 46 +/- 5 percent low-magnesium calcite even though free of gross detrital inclusions and boring or encrusting organisms. The calcite crystals occur in the interior of skeletal structures, have dimensions of 20 micrometers or less, and are surrounded by typical aragonite needles. Biogenic deposition seems to be the most likely source of the calcite, although the evidence does not rule out diagenesis of metastable.