We isolated a recessive Arabidopsis mutant, ctr1, that constitutively exhibits seedling and adult phenotypes observed in plants treated with the plant hormone ethylene. The ctr1 adult morphology can be phenocopied by treatment of wild-type plants with exogenous ethylene and is due, at least in part, to inhibition of cell elongation. Seedlings and adult ctr1 plants show constitutive expression of ethylene-regulated genes. The epistasis of ctr1 and other ethylene response mutants has defined the position of CTR1 in the ethylene signal transduction pathway. The CTR1 gene has been cloned, and the DNA sequences of four mutant alleles were determined. The gene encodes a putative serine/threonine protein kinase that is most closely related to the Raf protein kinase family.