Mediation of photoperiodic effects by indoleamines, especially melatonin, is known in higher vertebrates. A similar mechanism may occur in a unicellular alga, the dinoflagellate Gonyaulax polyedra. This organism entered the dormant stage of a cyst upon short-day treatment at lowered temperatures. Interruption of darkness by 2 hours of light prevented cyst formation, even when the overall duration of light was the same as in cyst-inducing short days. When given in a noninducing photoperiod, melatonin and an analog, 5-methoxytryptamine, substances that had previously been shown to occur in Gonyaulax, provoked cyst formation. Methoxylated indoleamines may play a role as mediators of darkness in this unicellular, in a similar way as in vertebrates, suggesting a common biochemical basis of photoperiodism.