The Dufour's gland of two solitary cavity-nesting bees, Osmia lignaria Say and Megachile rotundata (F.) (Hymenoptera: Megachilidae), was examined with microscopy to determine the structure and arrangement of the gland in the sting apparatus. The appearance and relative size of the Dufour's gland of these two bee species are similar. Unlike the termination of the Dufour's gland at the base of the sting in the honey bee, Apis mellifera L. ( Apidae), the posterior portion of the Dufour's gland in these megachilids enters into the sting bulb along with the venom duct. Within the sting bulb, the Dufour's gland is ventral to and longer than the venom duct. The following evidential findings presented here and elsewhere are in support of the hypothesis that the Dufour's gland is the source of an individual nest recognition cue in these two bee species: 1 ) the presence of a duct and exit pore at the posterior end of the Dufour's gland that may release glandular secretions, 2) the location of thick, brushy metasomal setae and the setosa membrane that could be used to apply a secretion to a substrate, and 3) the observed dragging of the tip of the abdomen during nest-marking.