Chromogranin B (CgB， also known as CHGB) is abundantly expressed in dense core secretory granules of multiple endocrine tissues and has been suggested to regulate granule biogenesis in some cell types， including the pancreatic islet β-cell， though the mechanisms are poorly understood. Here， we demonstrate a critical role for CgB in regulating secretory granule trafficking in the β-cell. Loss of CgB impairs glucose-stimulated insulin secretion， impedes proinsulin processing to yield increased proinsulin content， and alters the density of insulin-containing granules. Using an fluorescent pulse-chase strategy to track nascent proinsulin， we show that loss of CgB impairs Golgi budding of proinsulin-containing secretory granules， resulting in a substantial delay in trafficking of nascent granules to the plasma membrane with an overall decrease in total plasma membrane-associated granules. These studies demonstrate that CgB is necessary for efficient trafficking of secretory proteins into the budding granule， which impacts the availability of insulin-containing secretory granules for exocytic release.This article has an associated First Person interview with the first author of the paper.