Acetylcholinesterase (AChE) is a target enzyme of organophosphate (OP) and carbamate (CB) insecticides. Point mutation in AChE is one of the major mechanisms of OP and CB resistance. Recently, we investigated soluble AChEs abundantly expressed in the non-neuronal tissues of the honey bee Apis mellifera and fruit fly Drosophila melanogaster. Pre-incubation of OP and CB insecticides with honey bee soluble AChE showed a significant reduction in the inhibition of catalytic AChE activity. In the fruit fly, expression of soluble AChE was induced by insecticide exposure, and the wild-type fly expressing both soluble and membrane-anchored AChEs was more tolerant to insecticide than the transgenic fly expressing only membrane-anchored AChE. These findings suggested that soluble form of AChE is possibly involved in chemical defense against xenobiotics, including insecticides.