Field borders established for wildlife conservation have been recognized as a possible venue for also promoting beneficial insect populations, such as parasitic wasps and pollinators, on agricultural lands. However, traditional fallow field borders lack nectar sources required to sustain beneficial insect communities, and their value to small mammals is not well-understood. In October–November 2009, we trapped small mammals in four field-border treatments (planted native, warm-season grasses and prairie flowers, planted prairie flowers only, fallow vegetation, and frequently mowed vegetation) replicated around nine organic crop fields, and developed closed-population models in Program MARK to estimate abundance in each border. We also measured vegetation cover within each border treatment from June to August 2009. We captured 491 individuals of two species, the hispid cotton rat (Sigmodon hispidus) and house mouse (Mus musculus). Cotton rat abundance was ≥2 times greater in grass and flower borders and flowers-only borders than in fallow borders, likely because of greater vegetation density and availability of preferred foods in planted borders. No cotton rats were captured in mowed borders, and house mouse abundance was ≥5 times lower in mowed borders than in other border types. Lower abundance of cotton rats and house mice in mowed borders emphasizes the importance of structurally complex non-crop vegetation for supporting small-mammal communities in agricultural landscapes. Field borders planted to promote beneficial insects may be a useful tool for maximizing the ecological services provided by non-crop vegetation. © 2012 The Wildlife Society.