[Objectives]  To determine the effects of parasitism by Microplitis mediator (Haliday) on the feeding and development of Mythimna separata (Walker) larvae, and evaluate the potential of M. mediator as a biological control for M. separata. [Methods] The relative reduction in food consumption and development by M. separata larvae following parasitization by M. mediator was determined in controlled, laboratory experiments. [Results] 4, 6, and 8, day old M. separata larvae that had been parasitized by M. mediator consumed significantly less of an artificial diet than unparasitized larvae of the same age. The body weights of newly hatched, parasitized larvae increased slowly for 5 days then declined. No parasitized larvae pupated. The developmental duration of M. mediator was significantly shorter if these had parasitized older larvae. [Conclusion] M. mediator can not only reduce the damage caused by the current generation of M. separata but can also greatly reduce the abundance of this pest.