The foundation of marine coral reef ecosystems is calcium carbonate accumulated primarily by the action of hard corals (Coelenterata: Anthozoa: Scleractinia). Colonial hard coral polyps cover the surface of the reef and deposit calcium carbonate as the aragonite polymorph, stabilized into a continuous calcareous skeleton. Scleractinian coral skeleton composition and architecture are well documented; however, the cellular mechanisms of calcification are poorly understood. There is little information on the nature of the coral cell types involved or their cooperation in biocalcification. We report aragonite crystallization in primary cell cultures of a hard coral, Pocillopora damicornis. Cells of apical coral colony fragments were isolated by spontaneous in vitro dissociation. Single dissociated cell types were separated by density in a discontinuous Percoll gradient. Primary cell cultures displayed a transient increase in alkaline phosphatase (ALP) activity, to the level observed in intact corals. In adherent multicellular isolate cultures, enzyme activation was followed by precipitation of aragonite. Modification of the ionic formulation of the medium prolonged maintenance of isolates, delayed ALP activation, and delayed aragonite precipitation. These results demonstrate that in vitro crystallization of aragonite in coral cell cultures is possible, and provides an innovative approach to investigate reef-building coral calcification at the cellular level.