Lemna minor cultured in continuous light at constant temperature exhibits a seasonal rhythm in the rate of dry matter production with a maximum in summer and a minimum in winter. This confirms the observations of PIRSON and GöLLNER in 1953. Both the protein content and the protein/carbohydrate ratio are correlated with the rate of dry matter production and reach a maximum in summer. The contents of total and of water soluble carbohydrate, however, reach a maximum in winter. The seasonal changes in this periodicity under controlled conditions correspond to the seasonal changes in the periodicity under ecological conditions and exhibit the same dependence on water temperature as the latter. Therefore it is suggested that the seasonal rhythm of growth in Lemna minor is synchronized by water temperature.