Fecundity of the Japanese beetle (Popillia japonica Newman) was studied under expected future atmospheric concentrations of carbon dioxide. Mixtures of male and female Japanese beetles were fed soybean leaves grown either in an elevated carbon dioxide atmosphere (550 ppm) or a control atmosphere (370 ppm) for the duration of their adult lives. Cages were checked daily for adult beetle mortality, and laying media was sifted weekly to allow counting of eggs and larvae. Female beetles fed foliage from plants grown under elevated carbon dioxide laid significantly more eggs than control females. While female longevity did not differ between the treatments, male beetles fed foliage from plants grown under elevated carbon dioxide lived significantly longer than control males. Enhanced female egg production and extended mating opportunities due to increased male longevity may increase fecundity of Japanese beetles in soybean agroecosystems in elevated carbon dioxide conditions.