The architecture of subterranean nests of the ant Camponotus socius was studied from casts of plaster or metal. Twenty-four such casts are illustrated using stereo pairs of images. After study, plaster casts were dissolved to retrieve the workers embedded in them, providing a census of the ants that excavated the nest. Nests were up to 60 cm deep, and were composed of descending shafts connecting up to about 10 horizontal chambers. Nest volume ranged up to almost 800 cm³ and total chamber area up to almost 500 cm². Both volume and area were closely and positively related to the number of workers in the nest. Nest enlargement occurred through the simultaneous enlargement of chambers, deepening of the nest and addition of more chambers. Chamber enlargement contributed most to nest growth. Chambers near the surface were elongate and tunnel-like, while deeper chambers were more compact in outline. As chambers were enlarged, their outlines became more complex and lobed. Workers were polymorphic with clearly distinguishable minor and major workers. The headwidth of minors averaged 1.45 to 1.65 mm, and that of majors 2.30 to 2.80 mm. The mean headwidth of minors increased significantly as the proportion of major workers increased, but the trend of major headwidths fell short of significance. The numerical proportion of majors ranged from 3% to 38% and averaged 15% of the workers, while their biomass proportion ranged from 10% to 75%, averaging about 50%. The queen was recovered in 6 of the 14 plaster nests, suggesting that the average colony of this polydomous species has 2.3 nests. Because of the lateness of the season when casts were made (October), only two nests contained significant amounts of brood. The possible functional roles of nest architecture in ants are discussed.