A nion transport by the colonic mucosa maintains the hydration and pH of the colonic lumen, and its disruption causes a variety of diarrheal diseases. Cholinergic agonists raise cytosolic Ca²⁺ levels and stimulate anion secretion, but the mechanisms underlying this effect remain unclear. Cholinergic stimulation of anion secretion may occur via activation of Ca²⁺-activated Cl^– channels (CaCCs) or an increase in the Cl^– driving force through CFTR after activation of Ca²⁺-dependent K⁺ channels. Here we investigated the role of a candidate CaCC protein, bestrophin-2 (Best2), using Best2^–/– mice. Cholinergic stimulation of anion current was greatly reduced in Best2^–/– mice, consistent with our proposed role for Best2 as a CaCC. However, immunostaining revealed Best2 localized to the basolateral membrane of mucin-secreting colonic goblet cells, not the apical membrane of Cl^–-secreting enterocytes. In addition, in the absence of HCO₃^–, cholinergic-activated current was identical in control and Best2^–/– tissue preparations, which suggests that most of the Best2 current was carried by HCO₃^–. These data delineate an alternative model of cholinergic regulation of colonic anion secretion in which goblet cells play a critical role in HCO₃^– homeostasis. We therefore propose that Best2 is a HCO₃^– channel that works in concert with a Cl:HCO₃^– exchanger in the apical membrane to affect transcellular HCO₃^– transport. Furthermore, previous models implicating CFTR in cholinergic Cl^– secretion may be explained by substantial downregulation of Best2 in Cftr^–/– mice.