We examine the social characteristics and sex ratio of the recently described Anelosimus oritoyacu Agnarsson 2006. We find that this spider, whose nests occur on tree crowns and bushes in open fields near Baeza, Ecuador, lives in colonies that may contain from one to several thousand adult females and their progeny. It differs from most other social congeners in that it occurs at relatively high elevations (1800–1900 m) and its primary sex ratio, 2.5 females per male, is the least biased of any known social species in the genus. The low sex ratio bias may reflect a low colony turnover rather than high gene flow among colonies, as the colonies occurred in complexes that were few and far between, but appeared to be long-lived. The relatively small body size of adult females and a web that appears to allow the capture of insects from all directions, combined with individual and group foraging, may allow the formation of large colonies at an elevation where insects, albeit abundant, are for the most part small.