Bumble bees (Bombus Latr.) are primarily a cold temperate group, but a few species live in the hot, moist conditions of tropical rainforest. We describe the external and internal characteristics of a Bombus pullatus Franklin nest from the tropical lowlands of Costa Rica. The nest was a large, conical mound constructed of cut vegetation on the forest floor, similar to nests of the Amazonian bumble bee, B. transversalis (Olivier). Maintenance of the vegetative canopy involved moving cut materials with the mandibles and a backward-directed sweeping of the legs, behaviors reported in some species of Fervidobombus and Thoracobombus and in the sister species to B. pullatus, B. transversalis and B. atratus Franklin. We recorded foraging activity, task specificity, and internal parasites of bees at the nest. Foragers were polylectic and peak foraging rates, particularly for pollen, were during the morning. The nest had five active entrances, and foragers tended to have a unidirectional flow through these entrances. The colony, with 414 adult workers and both worker and reproductive brood, was larger than most temperate colonies and comparable in size to nests of other tropical bumble bee species. A few workers were parasitized by the conopid fly, Physocephala sp.