The locus of symbiotic dinoflagellates within host cells prorides a habitat which could potentially be exploited by the alga through heterotrophic uptake of host-derived organic substrates. Using zooxanthellae (S)'mbtodmmm sp ) isolated from the tropical sea anemone Alptasm pulchella collected from Kaneohe Bay, Hawan, the effect of various potential organic substrates on growth m vztro was assessed in Erdschrelber seawater medium (ES) supplemented with organic compounds. Zooxanthellae maintained at 5 to 7ktE m -2 s -1 (below compensation irradiance) grew heterotrophlcally when supplied with 100/~M glycerol, glycolate, acetate, malate, or propionate, and grew in darkness on 100ktM propionate. Zooxanthellae exposed to irradiance below compensation were able to utilize carbon sources m the unsupplemented ES medium for slow growth, but generally the growth rate of cultured zooxanthellae was a function of incubation irradiance Zooxanthellae incubated for 10 wk in unsupplemented ES at 5 to 7 gEm -2 s -1 were capable of growth at th~s low trradiance, but were also capable of net photosynthetic oxygen production at higher irradlances. This suggests that zooxanthellae can be photoautotrophlc or facultatively heterotrophlc An estimate for the duration of mitosis (td) is made on the basis of growth rate of cultured zooxanthellae in log-phase; this estimate of td= 4.88 h is less than half the estimated td for zooxanthellae in sltu