The uncovered, single-comb nests of Mischocyttarus baconi Starr have short, stout, centric petioles. We describe nest size, cell size, petiole dimensions, and the position of the petiole. In a sample of 91 nests, the comb comprised up to 198 cells. Cells had a mean side-to-side diameter at the mouth of 3.4 mm. The petiole had a mean length of 2.9 and a modal width of 2.0 mm. The degree of excentricity was determined for a sample of nests of at least 20 cells. As expected, those attached to vertical substrates (wall nests) showed greater excentricity than those attached to horizontal substrates (ceiling nests).