The SNARE proteins, syntaxin, SNAP-25 and synaptobrevin form a tertiary complex essential for vesicle fusion. Proteins that influence SNARE complex assembly are therefore likely to be important regulators of fusion events. In this study we have focused on tomosyn, a highly conserved, neuronally enriched, syntaxin-binding protein that has been implicated in the regulation of vesicle exocytosis. To directly test the role of tomosyn in neurosecretion we analysed loss-of-function mutants in the single Caenorhabditis elegans tomosyn gene, tom-1. These mutants exhibit enhanced synaptic transmission based on electrophysiological analysis of neuromuscular junction activity. This phenotype is the result of increased synaptic vesicle priming. In addition, we present evidence that tom-1 mutants also exhibit enhanced peptide release from dense core vesicles. These results indicate that tomosyn negatively regulates secretion for both vesicle types, possibly through a common mechanism, interfering with SNARE complex formation, thereby inhibiting vesicle fusion.