The neotropical bumble bee Bombus (Fervidobombus) atratus Franklin is widely dis- tributed in South America ranging from tropical and subtropical lowlands to high altitudes in the Andes. Most of its biology is known from studies conducted in Brazilian lowland forests and almost nothing is known from other areas, especially at high altitudes. Here we provide data on the nest architecture, brood development, worker behavior, seasonal cycle and associated organ- isms from seven colonies of B. atratus observed above 2000 m of altitude in Colombia and Ecuador. Then, we compare them with those data from Brazil. All colonies found were located above the ground, in disturbed areas. Most of the nests either lacked a defined entrance or had a single entrance; a single nest had five entrances, one of them more active than the others. Nests had from 1 to 8 active queens and up to 80 workers indicating monogynous and polygynous cycles as reported from the lowlands. Nests initially lacked an involucrum covering the brood but even- tually developed an irregular involucrum of wax mixed with cardboard and carcasses of B. atratus and their associated beetles (Antherophagus sp., Cryptophagidae). Bees also built pollen pockets attached to larval clusters for feeding larvae. The average developmental time from egg to adult (29.6 days) and the percentage of cells with two pollen pockets (63.6%) were significantly greater than those previously reported. The maximum pocket diameter was significantly smaller, about half of the size, than those diameters observed in lowland colonies. The ecological siwcance of such reduction in size is still unclear but could explain the higher frequency of cells with two pockets in our colony. Nests maintained an internal nest temperature about 12°C warmer than external environmental temperature. Several workers were observed constantly scraping and cut- ting litter on top of one of the nests. Previously this behavior had only been known in Bombus (Fervidobombus) transuersalis (Oliver), a closely related Amazonian species. As in the lowlands, B. atratus colonies at high altitudes seem to be active year-round. The beetle Antherophagus sp. was found in two of the seven colonies observed. They are probably scavengers, but nothing is cer- tainly known about their role within tropical Bombus colonies. Resumen.-El abejorro neotropical Bombus (Fervidobombus) atratus Franklin estA ampliamente distribuido en Sur AmQica, encontrAndose desde las tierras bajas tropicales y subtropicales hasta las grandes altitudes en 10s Andes. Gran parte de su biologia es conocida de estudios realizados en las tierras bajas brasileras y casi nada se conoce de otras Areas, especialmente a grandes alturas. Aqui proporcionamos datos sobre la arquitectura de 10s nidos, ciclo de desarrollo, comportamiento de las obreras, ciclo estacional y organismos asociados de siete colonias observadas a mAs de 2000 m de altura en Colombia y Ecuador. Luego, nuestros datos son comparados con 10s datos de Brasil. Todas las colonias encontradas estaban sobre el suelo, en Areas perturbadas. La mayoria de 10s nidos carecian de una entrada definida o presentaban una sola entrada; un solo nido tenia 5 entradas, una de las cuales era mAs activa que las otras. Los nidos tenian de una a ocho reinas