A 10-fold circadian variation in the amount of luciferin binding protein (LBP) in the marine dinoflagellate Gonyaulax polyedra is reported. This protein binds and stabilizes luciferin, the bioluminescence substrate. In early night phase, when bioluminescence is increasing and LBP levels are rising in the cell, pulse labeling experiments show that LBP is being rapidly synthesized in vivo. At other times, the rate of LBP synthesis is at least 50 times lower, while the rate of synthesis of most other proteins remains the same. The LBP mRNA levels, as determined by in vitro translations and by RNA (Northern) hybridizations, do not vary over the daily cycle, indicating that circadian control of bioluminescence in this species is mediated by translation.