The oriental armyworm Mythimna separata (Walker) is a devastating pest of cereal crops mainly in Asia and Oceania and recently become resistant to beta-cypermethrin (beta-CP). However, molecular biological studies of its response to beta-CP are scarce, and related genomic information is not available. In this study, we sequenced and de novo assembled the transcriptome of beta-CP susceptible M. separata (MsSwhole, abbr. MsS-W). A total of 30,486 unigenes were obtained, with an N50 length of 506 bp. A number of 21,051 unigenes were matched to public databases, of which 10,107 were classified into 59 gene ontology subcategories, 5792 were assigned into 25 clusters of orthologous groups of proteins subgroups and 12,123 were assigned to 257 Kyoto Encyclopedia of Genes and Genomes pathways. A total of 729 simple sequence repeats were detected. In addition, a total of 323 cytochrome P450-associated sequences from nine lepidopterous species, of which 130 were from M. separata, were analyzed using the maximum likelihood method and Bayesian inference. Among the 130 cytochrome P450-associated sequences from M. separata, 60 were dropped into CYP3 clan, which is associated with metabolizing xenobiotics and plant natural compounds. Furthermore, the beta-CP susceptible (MsS-2) and resistant (MsR-2) M. separata population transcriptomes were sequenced.Certain critical genes involved in beta-CP detoxification were