Continuous treatment with high doses of puromycin suppresses the circadian rhythm of bioluminescence inGonyaulax polyedra.When given in sub‐maximal doses, which are inhibitory only in the third or fourth cycle, the antibiotic causes phase delays. At a higher dosage, pulse treatment in different phases results in a phase response curve. Occurrence of phase advance shifts and extent of delay shifts depend on pulse duration. Continuous treatment with a falsifier of transcription, 5‐azacy tidine, causes a reduction in bioluminescence, but does not delay the rhythm. Therefore, the role of protein synthesis in theGonyaulaxclock seems to be predominantly of post‐transcriptional nature.