In order to investigate the role of a circadian clock in the timing of a monthly rhythm, different populations of planarians were maintained in constant darkness or, to manipulate the period of a circadian rhythm, in light‐dark cycles totalling 20 h, 24 h, or 28 h. The orientation response of the planarians was followed for 2 months. If solar‐ and lunar‐day rhythms were interacting to produce a monthly oscillation in the orientation response, the interval between “monthly”; peaks would be predictably altered. However, no difference in the phase or period of the monthly rhythm was observed among the experimental groups.