By screening suppressor mutants of the hy2 mutation of Arabidopsis thaliana, two dominant photomorphogenic mutants, shy1‐1D and shy2‐1D, for two genetic loci designated as SHY1 and SHY2 (suppressor of hy2 mutation) have been isolated. Both of these non‐allelic, extragenic suppressor mutations of hy2 are located on chromosome 1 of the Arabidopsis genome. Both mutations suppress the elongated hypocotyl phenotype of hy2 by light‐independent inhibition of hypocotyl growth as well as by increasing the effectiveness of light inhibition of hypocotyl elongation. The shy1‐1D mutation is partially photomorphogenic in darkness with apical hook opening and reduced hypocotyl elongation. The shy2‐1D mutant displays highly photomorphogenic characteristics in darkness such as true leaf development, cotyledon expansion, and extremely reduced hypocotyl growth. In regard to hypocotyl elongation, however, the shy2‐1D mutation is still light sensitive. Examination of red/far‐red light responses shows that the shy1‐1D mutation suppresses the hypocotyl elongation of the hy2 mutation effectively in red light but not effectively in far‐red light. The shy2‐1D suppresses hypocotyl elongation of the hy2 mutation effectively in both red and far‐red light. Both mutations can also suppress the early‐flowering phenotype of hy2 and have a distinct pleiotropic effect on leaf development such as upward leaf rolling. The data obtained suggest that SHY1 and SHY2 represent a novel class of components involved in the photomorphogenic pathways of Arabidopsis. This is the first report on the identification of dominant mutations in the light signal transduction pathway of plants.