The homotypic fusion of yeast vacuoles includes a ＇docking＇ step, which we show here to consist of two sequential reactions: a reversible ＇tethering＇ mediated by the GTPase Ypt7, and ＇SNARE pairing＇, in which SNARE proteins from opposite membranes form a complex in trans. The function of this trans-SNARE complex must be transient, as the complex can be disassembled by excess Sec18 in the presence of Sec17 and ATP without influencing the fusion rate. These data Indicate that SNARE pairing may transiently signal to downstream factors, leading to fusion.