Animals often acquire their microbial symbionts from the environment, but the mechanisms underlying how specificity of the association is achieved are poorly understood. We demonstrate that the conserved proton pump, V-type ATPase (VHA), plays a key role in the establishment of the model light-organ symbiosis between the squid Euprymna scolopes and its bacterial partner, Vibrio fischeri. Recruitment of V. fischeri from the surrounding seawater is mediated by juvenile-specific ciliated fields on the organs surface. These epithelia produce acidic mucus containing antimicrobials with low-pH optima, creating a chemical environment fostering specific recruitment of V. fischeri. We provide evidence that this critical acidic landscape is created by activity of VHA. VHA inhibition abolished epithelial-cell acidity, resulting in increased mucus pH and inefficient symbiont colonization. Thus, VHA provides a mechanistic link between host modulation of microenvironmental acidity, immune function, and selection of microbial symbionts, a strategy for specificity that may govern other symbioses.