Synergist S,S,S-tributyl phosphorotrithioate and piperonyl butoxide were used to determine the possible contribution of hydrolytic and oxidative metabolism to malathion and MIPC resistance in Nilaparvara lugens Stal. For malathion resistance, the role of microsomal oxidases was unclear, and for MIPC-resistance, it was generally limited. Esterase hydrolysis was closely involved in malathion resistance. Spectrophotometric and electrophoretic studies confirmed this association. This hydrolytic degradation also contributed significantly to the resistance to MIPC and several other carbamates, although to varying extents. Little difference was detected in malaoxon sensitivity of acetylcholinesterases (AChE) of susceptible (S) and malathion-resistant strains. However, compared with that of S strain, AChE of MIPC-resistant N. lugens was 15.7-fold less sensitive to MIPC, and also less sensitive to other carbamates to which this strain had developed various levels of resistance. Resistance to both malathion and MIPC in N. lugens was a partially dominant trait governed by more than one factor.