Size and frequency of division were determined for zooxanthellae from nine scleractinian coral species collected in February, 1983 at Discovery Bay, Jamaica, from four depths over a 51 m bathymetric range. Mean diameters of zooxanthellae ranged from 6.4 to 12.6 μm. Small zooxanthellae were found in Madracis mirabilis, Eusmilia fastigiata and Dendrogyra cylindrus whereas larger cells were seen in Agaricia agaricites, Porites astreoides, Montastrea cavernosa and Acropora cervicornis. Zooxanthellae division was not phased over a diel cycle. The percentage of zooxanthellae in a paired stage of cytokinesis (mitotic index or MI) was highly variable and ranged from 1.1% to 14.1%. Values measured in E. fastigiata and D. cylindrus were greater than in the other corals. MI was higher in branch tips of A. cervicornis than in branch bases. Daily average MI was negatively correlated with mean cell diameter and for a majority of the coral species increased with habitat depth. MI values were used to estimate specific growth rates and generation times for zooxanthellae in vivo for comparison with other dinoflagellates.