Heterotrophic dinoflagellates from Texas coastal waters were assayed for their bioluminescence capacity and the effects of light (photoinhibition) and starvation on their bioluminescence. The large, common heterotrophic dinoflagellate Noctiluca scintillans (Macartney) Ehrenberg was found to produce almost-equal-to 6 x 10(10) photons per cell. The bioluminescence of this dinoflagellate was not reduced by exposure to light, nor was there any detectable diel pattern of stimulable bioluminescence. Noctiluca fed a mixture of phytoplankton foods increased slightly in bioluminescence capacity over a 2-wk period, whereas cells held without food showed a steady decline in bioluminescence. Several heterotrophic dinoflagellates of the genus Protoperidinium were grown in laboratory enrichments and tested for their bioluminescence capacity. The bioluminescence potential for Protoperidinium species ranged from 0.6 x 10(10) photons per cell for an unidentified species of Protoperidinium, the smallest species we tested (56 x 50-mu-m), to 3 x 10(10) photons per cell for P. depressum (Bailey) Balech, the largest species tested (132 x 116-mu-m). Several species, including P. oblongum (Aurivillius) Parke et Dodge, P. hirobis (Abe) Balech, P. divaricatum (Meunier) Balech and P. steinii (Jorgensen) Balech, were found to be non-bioluminescent. All bioluminescent species of Protoperidinium were found to have their mechanically stimulable bioluminescence inhibited by exposure to light. In more detailed studies of photoinhibition in P. divergens (Ehrenberg) Balech, a light intensity of almost-equal-to 1.5 x 10(14) photons cm-1 s-1 was necessary for photoinhibition. Recovery of bioluminescence was complete within 30 min of transfer into darkness and complete photoinhibition occurred 10 min after placing cells in light of 1.5 x 10(16) photons cm-2 s-1. There appeared to be no diel pattern of mechanically stimulable bioluminescence in P. divergens. P. depressum held for 3 days without food showed a decrease in bioluminescence potential compared to cells that remained in the enrichments in which they were grown. P. depressum held in different food concentrations for 48 h showed no difference in bioluminescence capacity.