In this paper we provide a field account of some aspects of the behavioural biology of Goniosoma longipes (Roewer), a harvestman which commonly occurs in caves in South-east Brazil. During daytime, solitary and aggregated individuals can be found resting on the cave walls. Just after sunset, however, many individuals leave the cave to forage for live and dead arthropods. Foraging individuals return to the cave before dawn. Aggregations of G. longipes contain on average 34 individuals (range 7-200), and the groups are usually found close to the water source and away from the cave entrance. The main predators of G. longipes are the spider Ctenus fasciatus Mello-Leitao (Ctenidae) and the opossum Philander opossum (L.) (Didelphidae). Upon disturbance solitary and aggregated individuals may either flee, or drop from the cave wall or vegetation. The harvestmen can also release a repugnatory liquid upon manipulation, and aggregated individuals collectively discharge this secretion toward the aggressor before fleeing. The activity schedule of G, longipes shows that individuals need to leave the cave periodically to forage, and therefore the population can be considered trogloxene. Data on the food items collected by G. longipes indicate that the harvestman is a generalist predator which also feeds on dead animal matter. Gregarious behaviour is considered relatively common among harvestmen and has been interpreted in several ways. We suggest that gregarious behaviour in G. longipes may be related with the choice of more suitable microconditions in the cave habitat and/or with group chemical defence.344587596