The soil excavated from nest chambers ofTrachymyrmex septentrionalisMcCook is placed in crescent-shaped heaps to one side of the entrance hole. The crescents occupy a mean arc of about 75° (but this increases somewhat with time) at a mean distance of 8.0 cm from the hole; a new mound is built at the same orientation and distance if the original mound is removed. The immediate orientational cue which the excavating ants use is a visual image, perhaps landmarks, but not simply a positive or negative phototaxis. If the local slope is altered the mound will be gradually reoriented to a downslope direction over a period of many days. Mound orientation is highly individualistic for each nest, and the compass orientations often appear to be random among nests of a local population.