We have recently shown that during in vivo photoinhibition the D1 protein is degraded via a modified form, designated D1*. Depending on light conditions, the amount of D1* varies in leaves between 0 and 50% of total D1 content. By isolating thylakoids from leaves acclimated to different light levels, and performing photoinhibition experiments on these thylakoids, the following results on D1 protein degradation were obtained: (i) the protease involved in D1 degradation requires activation by light; (ii) neither acceptor nor donor side photoinhibition of PSII induces formation of D1* in vitro; (iii) in isolated thylakoids, the transformation of D1 to D1* can be induced in low light in the presence of ATP, which suggests that D1* is a phosphorylated form of the D1 protein; (iv) D1*, induced either in vivo or in vitro, is much less susceptible to degradation during illumination of isolated thylakoids than the original D1 protein. We suggest that the modification to D1* is a means to prevent disassembly of photodamaged photosystem II complex in appressed membranes.