Abstract: Bionomics of Chilo suppressalis larvae overwintering in paddy-rice and water-oat fields were investigated in Beijing in early-winter (middle November) and after-winter (early March). The results show that overwintering larvae are 4th instar or older, and the 6th instar ratio in water-oat fields is 17.5% higher than that in paddy-rice fields in early-winter. Old larvae account for a higher percent in after-winter period than in early-winter. Female to male larvae ratios are respectively 1︰2.0 and 1︰1.7 in early-winter and after-winter periods in paddy-rice fields, and 1︰1.4 and 1︰1.2 in water-oat fields. Larvae duration is longer in early-winter than after-winter, and not significantly different between in paddy-rice and water-oat fields in one period. However, larvae from water-oat fields take 14.0% more time before pupation than that from paddy-rice fields in early-winter, while larvae and pupa from water-oat fields are significantly heavier than those from paddy-rice fields. Mortality rate of larvae collected in early-winter is greater than 65%. Fungi infection is the primary mortality factor in water-oat fields (i.e. over 90% total mortality), while parasitic wasp and bacteria infection constitute the primary mortality factors in paddy-rice fields (i.e. respectively over 50% and 25% total mortality).Apanteles chilonis and Eriborus terebrans are the two dominant parasitic wasps.