The amount of the luciferin (substrate) binding protein (LBP) in extracts of the bioluminescent marine dinoflagellate Gonyaolax polyedra increases by about 10-fold during a 6-hour period spanning the end of the day- and the early night-phases, and then decreases by the same amount approximately 12 hours later. Previous studies have indicated that synthesis of the protein is regulated translationally. The experiments described here were undertaken to gain insight into the mechanism of this control. Evidence was obtained for the existence of mRNA-binding proteins that could have a regulatory function. In addition, circadian-related differences were observed in the total amount of mRNA as well as in the subcellular distribution of lbp mRNA.