Brassinosteroids are essential for plant growth and development. Brz is a specific inhibitor of the brassinosteroid biosynthesis. We had identified Brz insensitive mutant bill from EMS-mutation lines, and bil5 from fast neutron-mutation lines. To identify additional components of the brassinosteroid signaling, trial to screen new mutants as Brz-insesitive-long hypocotyls in the dark was applied to Arabidopsis activation-tag lines. The semidominant mutant bssl (Brz-sensitive-shortl) showed normally etiolated hypocotyl same as wild type without Brz in the dark. However, with Brz, bssl hypocotyl became about 70% length of wild type in the dark. In the weak light, hyocotyl of bssl showed about 50% length of wild type without Brz. These results suggested that bssl hypocotyl is hypersensitive to Brz and the light. In the later growth stage, bssl showed extremely dwarf phenotype by inhibition of axis elongation. An activation tag insertion was identified in non-ORF region on bssl genome. Further analysis is in progress.