The alfalfa leafcutting bee, Megachile rotundata (F.) (Hymenoptera: Megachilidae), is widely used in western North America to enhance pollination of alfalfa, Medicago sativa L., grown for seed production. Feral populations have been documented but have been little-studied, We used trap-nests of a wide range of diameters (3–9 mm) to study a feral population at a wildflower seed farm in Montana. Adult females accepted all hole sizes but tended to provision nest tunnels of smaller diameters (3–4 mm) than those provided in managed populations (5–7 mm). Progeny, especially females, from intermediate- and large-diameter nests averaged larger body size that those in the two smallest nest-diameter classes. Offspring were often larger in nests with greater numbers of offspring, indicating that there was no trade-off between offspring size and number within nests. Individual nests tended to contain a relatively small range of progeny sizes and those sizes tended to vary among nests of the same diameter. Previous studies of M. rotundata indicate that heritability of body size is low, most offspring within nests are full siblings, and variation in offspring size is due to variation in the amount of provision provided by mothers. Thus, the fact that body size varied among families probably related to variation in provisioning capabilities among females. We also found evidence that offspring size varied spatially but that seasonal trends in offspring size were weak. Sex ratio varied among different nest sizes, but only the smallest and largest nests produced biased ratios. Sex ratios were less male-biased than in commercially managed populations of M. rotundata.