Environmental light signals are sensed by multiple families of photoreceptors and transduced by largely unknown mechanisms to regulate plant development. In this report, genetic analysis suggested that light signals perceived by both phytochromes and a blue light receptor converge to repress the action of Arabidopsis COP9 in suppressing seedling photomorphogenesis. Molecular cloning of the gene revealed that COP9 encodes a novel protein of 197 amino acids whose expression is not regulated by light. COP9 functions as a large (> 560 kDa) complex(es) that is probably subjected to light modulation. In addition, COP8 and COP11 are required for either the COP9 complex formation or its stability. Therefore COP9, together with COP8 and COP11, defines a novel signaling step in mediating light control of plant development.