Several lines of evidence favour the hypothesis that intracellular biosynthetic protein transport in eukaryotes is mediated by non‐clathrin‐coated vesicles (for a review see Rothman and Orci, 1992). The vesicles have been isolated and a set of their surface proteins has been characterized as coat proteins (COPs). These COPs exist in the cytosol as a preformed complex, the coatomer, which was prior to this study known to contain six subunits: four (alpha‐, beta‐, gamma‐ and delta‐COP) with molecular weights between 160 and 58 kDa, and two additional proteins of approximately 36 and 20 kDa, epsilon‐ and xi‐COP. Here we describe a novel subunit of the coatomer complex, beta′‐COP. This subunit occurs in amounts stoichiometric to the established COPs both in the coatomer and in nonclathrin‐coated vesicles and shows homology to the beta‐subunits of trimeric G proteins.