In phosphate deficient Lemna minor photosynthesis is decreased. The plants show a rapid increase in photosynthesis upon P-supply, often not only at saturating light intensities, but also at rate limiting intensities. The reactivation at light limitation is larger in white light than in far red, and as far as monochromatic light has been studied, larger at 650 nm than at 700, 711 and 717 nm, which probably depends on a stronger inhibition of photosynthesis at shorter wavelengths. The results are interpreted as demonstrating an effect of the P-level on electron transport to and from photosystem I, in which the coupling to photophosphorylation and a switch from cyclic to pseudocyclic photophosphorylation may be involved.