The population dynamics and key mortality factors of the Oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae), a serious pest of cereal crops in Asia and Australia, were studied in southern India. Adults were generally caught in light traps 15-20 days after the initiation of the monsoon rains in the first week of June, and reached a peak in September, nearly one month after the peak in larval density. Rainfall, and maximum and minimum relative humidity were positively associated moth catches in the light traps, while maximum temperature, open pan evaporation, solar radiation, sunshine hours, and wind velocity showed a negative correlation with moth abundance. Stepwise regression analysis of moth catches with weather conditions over the previous 2 and 4 weeks explained 54-68% of the variation in the number of moths caught in the light traps. Five hymenopteran parasitoids [Cotesia ruficrus (Haliday), Metopius rufus Cameron, Disophyrys sp., Campoletis chlorideae Uchida, Enicospilus sp.], and five dipteran parasitoids [Carcelia illota (Curran), Sturmiopsis inferens Townsend, Palexorista solennis (Walker), P. laxa (Curran), and Megasellia sp.], mermithid (Neoplectana sp.), and nuclear polyhedrosis virus (NPV) regulated its populations under natural conditions. Parasitism levels were much greater in sorghum (34.6%) than in pearl millet (17.6%). Cotesia ruficrus was the principle mortality factor, which caused up to 47% parasitism in October. Its activity was greater in sorghum (24.6%) than in pearl millet (14.9%). This parasitoid species could be exploited for the biological control of this pest. none