Background: Rice is the most important food crop of Arunachal Pradesh and Jhum rice constitutes a major traditional cultivation system. Infestation of armyworm, Mythimna separata (Walker) was noticed in the form of an outbreak in several villages of West Siang and Lepa Rada districts of Arunachal Pradesh in the first week of July 2017. Aim: The aim of this study was to assess the severity of the crop infestation during the outbreak and also the cause of its natural suppression. Objectives: The study was conducted with the objective to record the level of infestation of M. separata in Jhum rice along with the association of natural enemies, if any. Material: Surveys were conducted to record the level of infestation in the field. Observations were recorded on the number of larvae per plant as well as number of plants infested. Result: The level of infestation of paddy was recorded as 89.67±0.03 per cent with 6.37±0.61 larvae per plant which after 10 days, reduced to 78.7±0.03 per cent with 5.34±0.23 larvae per plant. Further, it dropped down to 14.7±0.05 per cent with 0.17±0.06 larvae per plant and ultimately to 6.3±0.025 per cent infestation with 0.07±0.03 larvae per plant. A braconid, Cotesia rufricus and a dipteran parasitoid were also found parasitizing the larvae of M. separata. Conclusion: The armyworm, M. separata has the tendency of emerging in the form of outbreaks causing heavy crop infestation. The parasitoids identified in the study could be of immense importance in managing such an outbreak in future.