In growing cultures of the dinoflagellate, Gonyaulax polyedra, total protein synthesis showed a circadian rhythm with a maximum during the phase of the cycle which corresponded to the previous darktime. The maximum coincided with the maximal phase shift of the glow rhythm caused by lower concentrations of the antibiotic anisomycin (Taylor, W., et al., 1982). J. Comp. Physiol. 148 B, 11-25. The dose reponses of inhibition of protein synthesis correlated well with the phase shifting by anisomycin. The amplitude and level of the total-protein synthesis rhythm increased with the growth rate, indicating that the majority of proteins controlled by the circadian clock were cell cycle-dependent. The degradation rate showed the same circadian rhythm as the synthesis rate. Slight variations in uptake and pool size of amino acids were not responsible for the rhythm in the protein-synthesis rate.