Membrane fusion is necessary both in the eukaryotic secretory pathway and for the inheritance of organelles during the cell cycle. In the secretory pathway, heterotypic fusion takes place between small transport vesicles and organelles. It requires N-ethylmaleimide-sensitive fusion protein (NSF/Sec18p), soluble NSF attachment proteins (SNAPs/Sec17p) and SNAP receptors (SNAREs). SNAREs are integral membrane proteins (v-SNAREs on vesicles, t-SNAREs on the target organelles) and are thought to provide specificity to the fusion process. It has been suggested that Sec17p and Sec18p bind to v-SNARE/t-SNARE complexes and mediate the membrane fusion event. Homotypic fusion of yeast vacuoles also requires Sec17p and Sec18p (ref. 6), but in vitro they are needed only to 'prime' the vacuoles, not for subsequent docking or fusion. It has been unclear whether these reactions involve SNAREs that are similar to those previously identified in heterotypic fusion systems and, hence, whether the actions of Sec18p/NSF and Sec17p/alpha SNAP in these systems can be compared. Here we identify typical v- and t-SNAREs on the yeast vacuolar membrane. Although both are normally present, vacuoles containing only the v-SNARE can fuse with those containing only the t-SNARE. Vacuoles containing neither SNARE cannot fuse with those containing both, demonstrating that docking is mediated by cognate SNAREs on the two organelle membranes. Even when t- and v-SNAREs are on separate membranes, Sec17p and Sec18p act at the priming stage. Their action is not required at the point of assembly of the SNARE complex, nor for the fusion event itself.