Phosphorylation of cellular proteins is a key regulatory mehanism for signal transduction pathway in living cells. Phytochrome, a red/far-red light photoreceptor in plants, is known to employ protein phosphorylation for its light signaling, although its detauked mechanism is still ambiguous. This study is intended to identify the phosphoproteins and protein kinases that are regulated by phytochrome, by employing transgenic rice seedlings that overexpress Arabidopsis phytochrome A. Red light stimulated phsophorylation of a 70 kDa protein and far-red light negated the effect. The red light induced phosphotylation of the 70 kDa protein was strongly activated by heparin and inhibited by poly-L-lysine, suggesting that the 70 kDa protein phosphorylating kinase belongs to GSK-3/SHAGGY protein kinase that has functional roles in establishing cell fate and pattern formation in Drosophila. Taken together with the fact that phytochrome controls plant development, these results may suggest that a GSK-3/SHAGGY-related protein kinase in plant(ASK) is likely to be involved in phytochrome signal transduction.