The unicellular alga Acetabularia was found to show a diurnal rhythm in photosynthesis. This rhythm continued for at least three cycles in constant light and temperature, and hence can be considered endogenous. Plants from which the nucleus had been removed by severing the basal rhizoids showed no modification in the photosynthetic rhythm over a number of cycles. The nucleus is, therefore, not immediately essential for the maintenance of rhythmicity in Acetabularia. Conversely, a mechanism for sustaining time-keeping must exist in the cytoplasm.