Reproductive potentials were compared for a large invasive lady beetle, sevenspotted lady beetle, Coccinella septempunctata L. (Coleoptera: Coccinellidae), and a set of four smaller native North American species that have been displaced from alfalfa, Medicago sativa L., fields in Utah. The invader rapidly attained predominance in these fields during years when aphid populations were high. In a laboratory experiment, females were provided with excess numbers of their principal prey in these fields, pea aphids, Acyrthosiphon pisum (Harris). Among the five species, both the number and total volume of eggs (number × mean egg volume) produced per day increased with increasing female size and were greatest for C. septempunctata. Rates of reproduction also increased with increasing female size within species. Similarly sized females of C. septempunctata and transverse lady beetle, Coccinella transversoguttata richardsoni (Brown), laid similar total volumes of eggs per day, but females of the invasive species had more ovarioles and laid larger numbers of individually smaller eggs. In summary, when feeding on abundant prey in a habitat in which it predominates, the invasive C. septempunctata gains reproductive advantage over native, North American lady beetles, from its large body size and its investment in many small eggs. The results support the generalization that high fecundity linked with large body size may often be one important factor that promotes the dominance of introduced species over native competitors in resource-rich environments.