With main photoperiods of red or white light, the inhibition of flowering in Lemna perpusilla 6746 caused by interruptions of the night with red light cannot be reversed by far-red light, since far-red light itself is highly inhibitory. However, with a main photoperiod of blue light, far-red light is much less inhibitory and partially reverses the effect of red night-interruptions. If the main blue photoperiod is terminated by a brief red exposure, reversibility is abolished, as the far-red light is again fully inhibitory. This latter effect can be reversed by far-red light. These results add light quality to the already known characteristics of the main light period which affect reversibility in the dark period, and are consistent with the idea that the effects of blue light on photoperiodism in L. perpusilla are mediated exclusively by phytochrome.