Abstract. Four tunnels off the lumen of the proventriculus of the carpenter bee, Xylocopa virginica virginica, have been discovered. Light and electron micrographs revealed the presence in the tunnels of gram‐positive, rod‐shaped bacteria, some of which showed division stages. The occurrence of the bacteria exclusively in the tunnels—not in the lumen of the proventriculus—makes speculation as to their function difficult. The function of the tunnels is also obscure, as no glands were seen emptying into them. The bacteria may be symbionts that release enzymes into the tunnels, which are subsequently transported to the midgut to aid digestion. They could also be parasites. This is the first report of such bacterial‐containing tunnels in insects.