*At the end of the cell cycle, the plant cell wall is deposited within a membrane compartment referred to as the cell plate. Little is known about the biogenesis of this transient membrane compartment. *We have positionally cloned and characterized a novel Arabidopsis gene, CLUB, identified by mutation. *CLUB/AtTRS130 encodes a putative TRAPPII tethering factor. club mutants are seedling-lethal and have a canonical cytokinesis-defective phenotype, characterized by the appearance of bi- or multinucleate cells with cell wall stubs, gaps and floating walls. Confocal microscopy showed that in club mutants, KNOLLE-positive vesicles formed and accumulated at the cell equator throughout cytokinesis, but failed to assemble into a cell plate. Similarly, electron micrographs showed large vesicles loosely connected as patchy, incomplete cell plates in club root tips. Neither the formation of KNOLLE-positive vesicles nor the delivery of these vesicles to the cell equator appeared to be perturbed in club mutants. Thus, the primary defect in club mutants appears to be an impairment in cell plate assembly. *As a putative tethering factor required for cell plate biogenesis, CLUB/AtTRS130 helps to define the identity of this membrane compartment and comprises an important handle on the regulation of cell plate assembly.