At two locations in central New York, including the Montezuma National Wildlife Refuge, Isodontia mexicana females nested in 8 and 9 mm diameter trap nests. As is typical for Isodontia, partitions and plugs in the nests were constructed using fragments of grass stems, and the outermost portion of the final nest plug consisted of a tuft of grass leaves that extended as far as 7 cm beyond the nest entrance. The vast majority of prey were adult tree crickets (Gryllidae: Oecanthinae; Oecanthus), although nymphal tree crickets and katydids (Tettigoniidae: Conocephalinae) also occurred among provisions. The sex ratio of adult tree cricket prey was strongly female-biased. Nest associates included three other nest-provisioning aculeates, along with Psocoptera, spiders, ants, Coelioxys sp. (Megachilidae), and bombyliid, sarcophagid, and phorid flies. The overall sex ratio of I. mexicana offspring did not differ from unity, but the sex of offspring in any given cell was related to the position of the cell and the total number of cells in a nest. As females added cells to the linear sequence in each nest, new cells were increasingly likely to house sons, especially in nests with fewer cells. Surprisingly, given results of previous studies, the sex of an offspring was unrelated to nest tunnel diameter.