Pharbitis nil, strain Violet, subjected to various photoperiods (24-hr cycle at 24°C) bloomed about 10 hr after light-off when the light period was 10 hr or longer, and about 20 hr after light-on when the light period was shorter. The higher the temperature (20-30°C) during the dark period, the later the time of flower-opening, with the temperature during the last half of the dark period having a stronger effect than that during the first half.In continuous dark or light, flower buds of Pharbitis opened about every 24 hr at all temperatures tested between 20 and 28°C, which suggests the participation of a circadian rhythm in determining the time of flower-opening. A light pulse given 6-12 or 28-36 hr after the onset of the dark period greatly advanced the phase of this rhythm (8-10 hr). Phase delay of this rhythm could not be obtained by light pulses given at any time. © 1979 Oxford University Press.