Understanding the balance between yield benefits and possible underlying yield costs that are associated with transgenic cultivars is useful for evaluating crop performance and the fitness of crop-wild hybrid progeny, but few researchers have tested for such costs under rigorous experimental conditions. We examined shifts in net costs and benefits of insect-resistance transgenes in cultivated rice (Oryza sativa) using two levels of insect pressure (low vs. moderate) and two types of competition (pure vs. mixed lines). We compared the growth and fecundity of potted rice plants from three transgenic lines, Bt, CpTI, and Bt/CpTI, relative to isogenic control plants at outdoor locations in Fuzhou, China. Net yield costs were detected, but only in Bt/CpTI plants in mixed-line competition with low insect pressure. These plants produced 16% fewer tillers, 6% smaller seeds, and 30% fewer seeds than competing control plants. Under moderate insect pressure, Bt and Bt/CpTI plants produced 36?65% more seeds than controls, but the net benefit for Bt/CpTI plants disappeared in mixed-line competition pots. To our knowledge, this is the first report of yield costs in cultivars with transgenic insect resistance. Our results suggest that these costs may be negligible in monotypic rice fields, especially when target insects are abundant.