The level of phytochrome in Lemna minor, grown in darkness interrupted by short during illuminations on a medium containing sucrose, is strongly affected by quality and timing of these illuminations. Red light induces a decrease in phytochrome. During a subsequent dark period, there is a recovery persisting for several days. This recovery was found to be independent of the initial degree of reduction, caused by the illumination. It is also insensitive to auxins and actinomycin D. Phytochrome could also be demonstrated spectrophotometrically int Letnna perpusilla. Lemna gibba and Spirodela intermedia.