Over a five year period, 419 nests of Mischocyttarus collarellus were found on 128 structures in and around La Selva biological research station in Costa Rica. The nest of this species is different from that of many other independent-founding polistines, in that it has a distinctly horizontal petiole and outward-facing cells (rather than a vertical petiole and downward-facing cells). Nests were sometimes situated on vertical surfaces, but were most often attached to the underside of a substrate leaning up to 20° from vertical. A few nests (about 6%) were found on buildings, but most were found on trees with smooth, peeling, or flaking bark. The four tree species found most commonly to bear nests were Psidium guajava, Cecropia obtusifolia, Vochysia guatemalensis, and Bactris gasipaes. Nests were found on tree trunks and primary branches, never on foliage or twigs. Median nest heights above ground were 250–350 cm, ranging from under 50 to over 850 cm. A systematic search of trees in an area of known high nest density demonstrated that even experienced searchers may miss at least 27% of nests in a given area. Nests were never found to reach densities greater than 54 nests/hectare or occupy more than about 12% of apparently possible nesting structures, even in a stand of a preferred tree species.