We investigate the Ca(2+)-triggered fusion of lipid vesicles site-selectively tethered to a DNA-modified supported lipid bilayer array, with the DNA strands designed such that hybridization occurs in a zipperlike fashion. Prior to the addition of Ca(2+), which is observed to induce docking and subsequent fusion (within 200 ms), the vesicles display lateral mobility determined by the number of tethers. Fusion is observed to require around ten DNA strands per vesicle, but does not occur at higher DNA coverage. However, despite the fact that fusion was restricted to occurring for vesicles tethered with around ten DNA strands, there is no correlation between single-vesicle diffusivity and fusogenicity. A possible scenario for the DNA-induced fusion machinery, consistent with these observations, is that prior to Ca(2+)-induced docking, the vesicles diffuse with a small number (2-4) of DNA tethers. Upon addition of Ca(2+), the vesicles dock, presumably due to bridging of lipid head groups. Fusion then occurs under conditions where 10-16 DNA tethers form and rearrange at the rim of the contact region between a docked vesicle and the SLB. The time required for this rearrangement, which may include both DNA hybridization and dehybridization during zipping, is expected to represent the observed docking and fusion time of less than 200 ms.