Turnip leaves infected with cauliflower mosaic virus accumulate variable amounts of a nonvirion protein of 66,000 MW. Tryptic fingerprint analysis showed that this protein is probably not related to the major virus coat protein (37,000 MW). RNA extracted from infected leaves directs the synthesis of a 66,000 MW protein, P66, in a wheat germ protein-synthesizing system. P66 synthesis in vitro could be arrested by prior hybridization of infected leaf RNA to viral DNA restriction fragments indicating that P66 is encoded by the virus genome and specifically by EcoRI fragment b. RNA coding for P66 sediments at about 18 S, binds, in part, to oligo dT-cellulose, and is sensitive to 7-methyl-GTP inhibition.