A study was made of the photoperiodic induction of flowering in Lemna minor, strain 6573. For comparison some experiments were performed with L. gibba. Like the latter, L. minor, strain 6573, proves to be a typical long-day plant. The critical daylength is about 10.5 hrs in a 24 hrs cycle, the response being, however, very weak. Much better responses are obtained with photoperiods of 13 and 14 hrs (from 25 to 30 per cent flowering). The response to a 13 hrs photoperiod shows a peculiar rhythm in the rate of development of flowers with periods of slow development alternating with periods of rapid development. A possible relation of this phenomenon to the observed rate of multiplication is discussed.