Phosphothreonine antibody was used to explore reversible thylakoid protein phosphorylation in vivo in evolutionally divergent organisms with oxygenic photosynthesis. Three distinct groups of organisms were found. Cyanobacteria and red algae, both with phycobilisome antenna system, did not show phosphorylation of any of the photosystem II (PSII) proteins and belong to group 1. Group 2 species, consisting of a moss, a liverwort and a fern, phosphorylated both the light-harvesting chlorophyll alb proteins (LHCII) and the PSII core proteins D2 and CP43, but not the D1 protein. Reversible phosphorylation of the D1 protein seems to be the latest event in the evolution of PSII protein phosphorylation and was found only in seed plants, in group 3 species. Light-intensity-dependent regulation of LHCII protein phosphorylation was similar in group 2 and 3 species, with maximal phosphorylation of LHCII at low light and nearly complete dephosphorylation at high light.