Obtusifoliol 14 alpha -demethylase is a plant orthologue of sterol 14 alpha -demethylase (CYP51) essential in sterol biosynthesis, We have prepared CYP51 antisense Arabidopsis in order to shed light on the sterol and steroid hormone biosynthesis in plants. Arabidopsis putative CYP51 cDNA AtCYP51 was obtained from Arabidopsis expressed sequence tag (EST) library and its function was examined in a yeast lanosterol 14 alpha -demethylase (Erg11) deficient mutant. A recombinant AtCYP51 protein fused with a yeast Erg11 signal-anchor peptide was able to complement the erg11 mutation, which confirmed AtCYP51 to be a functional sterol 14 alpha -demethylase, AtCYP51 was then used to generate transgenic Arabidopsis by transforming with pBI vector harboring AtCYP51 in the antisense direction under CaMV35S promoter. The resulting transgenic plants were decreased in accumulation of AtCYP51 mRNA and increased in the amount of endogenous obtusifoliol, They showed a semidwarf phenotype in the early growth stage and a longer life span than control plants. This newly found phenotype is different from previously characterized brassinosteroid (BR)-deficient campesterol biosynthesis mutants.