The cystic fibrosis gene encodes a cyclic AMP-gated chloride channel (CFTR) that mediates electrolyte transport across the luminal surfaces of a variety of epithelial cells^,,,. The molecular mechanisms that modulate CFTR activity in epithelial tissues are poorly understood. Here we show that CFTR is regulated by an epithelially expressed syntaxin (syntaxin 1A), a membrane protein that also modulates neurosecretion^,, and calcium-channel gating^,,, in brain. Syntaxin 1A physically interacts with CFTR chloride channels and regulates CFTR-mediated currents both in Xenopus oocytes and in epithelial cells that normally express these proteins. The physical and functional interactions between syntaxin 1A and CFTR are blocked by a syntaxin-binding protein of the Munc18 protein family (also called n-Sec1; ,,). Our results indicate that CFTR function in epithelial cells is regulated by an interplay between syntaxin and Munc18 isoforms.