In this study, acetylcholinesterase and choline oxidase were co-immobilized on poly(2-hydroxyethyl methacrylate) membranes and the change in oxygen consumption upon aldicarb introduction was measured. Immobilization of the enzymes was achieved either by entrapment or by surface attachment via a hybrid immobilization method including epichlorohydrin and Cibacron Blue F36A activation. Immobilized enzymes had a long-storage stability (only 15% activity decrease in 2 months in wet storage and no activity loss in dry storage). Aldicarb detection studies showed that a linear working range of 10-500 and 10-250 ppb aldicarb could be achieved by entrapped and surface immobilized enzymes, respectively. Enzymes immobilized on membrane surfaces responded to aldicarb presence more quickly than entrapped enzymes. Aldicarb concentrations as low as 23 and 12 ppb could be detected by entrapped and surface immobilized enzymes, respectively, in 25 min.