Metabolism of $^3H$-castasterone in the presence of $^{32}P$-ATP was examined by an enzyme solution prepared from A. thaliana after a reversed phased HPLC, after which a polar metabolite labeled by both $^3H$ and $^{32}P$ was obtained, suggesting that $^3H$-CS is phosphorylated by $^{32}P$-ATP. To elucidate the structure of the phosphorylated CS, the same enzyme assay was carried out with non-isotopes labeled CS and ATP. In GC-MS analysis the metabolite gave a molecular ion at m/z 664 as a bismethanboronate, suggesting the metabolite is a CS phosphate. Treatment of wheat germ acid phosphatase that hydrolyzed phosphoester bond gave the same mass spectrum and GC retention time in GC-MS analyses, confirming that the metabolite is phosphorylated CS. This is the first example of phosphorylated conjugates of CS in plants.