Abstract Nest site selection is a frequent context for decision making in ants, but little is known of the criteria used to make a choice. We tested the nest site preferences of Leptothorax curvispinosus, both by measuring hollow acorn nests occupied in nature, and by inducing laboratory colonies to choose between artificial nests of different design. Three criteria were examined. (1) Entrance size: the ants preferred small entrance holes, presumably for their greater defensibility and crypsis. Natural nest entrances were small, and 52% of them were reduced still further by the addition of rims of soil and leaf litter. In choice tests, colonies selected nest entrances near the median size of rimmed natural holes, rejecting those near the larger end of the distribution of raw natural holes. (2) Cavity volume: acorn cavity volume was weakly correlated with the size of the occupying colony. In choice tests, colonies rejected cavities near the median size of natural nests, preferring instead larger cavities near the upper end of the natural size distribution. This may reflect active size matching of colonies to nests, because the colonies used in the choice test were bigger than those from the natural nest sample. Alternatively, all colonies may prefer big nests, but face limited availability of large cavities in nature. (3) Cavity shape: colonies preferred shapes roughly similar to that of an acorn interior, rejecting thin crevices in favour of compact, high-ceilinged cavities.