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Nest architecture plays a fundamental role in the adaptation of ants to their habitat, favoring the action of economically important species. Ectatomma ruidum sp. 2 (ruidum species complex) is a biological control agent in Neotropical agroecosystems, exhibiting high biotur bation impact due to high nest densities. The architecture and composition of 152 nests were studied in two Andean populations of southwestern Colombia, 24 of them being cast using the paraffin wax technique. Nest entrance was a single, circular, 4 mm hole at ground level, without any special external structure, connected to a single vertical tunnel communi cating with successive half ellipsoidal chambers. Nests were extremely shallow (depth range: 28.7–35.4 cm), with an average of six chambers and an overall volume of 92.2 cm³ per nest. The deeper the chamber, the smaller its volume. Nest building was independent of plants or roots, and no surface or underground physical connections were found between neighboring nests. Few nests possessed a queen, and neither ergatoids nor microgynes were recorded. Despite significant interactions between localities and the number of both males and workers, queen presence had an overall highly positive effect on the number of workers and larvae and a negative one on the number of gynes. Overall, the studied Colom bian populations of E. ruidum sp. 2 retained the simple nest structure described for other species of this species complex and for colonies of the same species from other geographi cal areas, though they constrasted in their extreme shallowness. Our data suggest that E. ruidum sp. 2, at the local level, does not follow the usual monodomic pattern of this species with facultative polygyny but, rather, has a polydomic pattern with monogyny, perhaps related to the extreme shallowness of the nests due to soil structure, which could signifi cantly enhance the queen’s reproductive inhibition previously reported for this species. Inglés