Brassinosteroids are phytohormones that play important roles in plant growth, development and chloroplast differentiation. Recently, Brz (brassinazole) was synthesized as a first specific inhibitor of brassinosteroid biosynthesis. For analyzing the detail of mechanism of brassinosteroid biosynthesis and signal transduction, we started to screen for mutations that show resistance to the Brz actions. In genetic screening of Arabidopsis 140,000 seeds mutagenized by EMS and fast neutron, some mutants that were significantly taller than wild type when grown in the dark with Brz were screened. At least four mutants of dark grown with 3 μM Brz showed long hypocotyl as tall as wild type of dark grown in Brz-free condition. These are named as bil (B__-rz-i__-nsensitive-l__-ong hypocotyl). Another three mutants, that showed shorten hypocotyl but could not caused cotyledon-opening when grown in the dark with 3 μM Brz, were also screened. These are named as bih (B__-rz-i__-nsensitive-h__-ooked hypocotyl).