Biochemical assays to determine the susceptibility of malaria?'vectors to organophosphate and carbamate insecticides had been conducted in Central Java and Yogyakarta Provinces The objectives of this study were (1) to determine the susceptibility status of malaria vector larvae to organophosphate and err hamate and (2) to investigate the presence of two biochemical resistance mechanisms, possibly related to elevated esterase (non-specific esterase) and insensitive acetylcholinesterase. The research methods used were biochemical assays (microplate assay's) for elevated esterase and insensitive acetylcholinesterase. The esterase activity and insensitive acetylcholinesterase were measured at 450 nm and 405 nm with a Dytech EIASA plate reader. Biochemical assays indicated that wild population of malaria vector collected from Central Java and Yogyakarta Provinces were mostly decreased in susceptibility (resistant or tolerance), although there were different level of resistance present and different mechanism occurs. The percentage resistance of Anopheles maculatus ranged from 6,25 % to 27,08 % due to elevated esterase activity mechanism. There was no evidence of insensitive acetylcholinesterase mechanism in An. maculatus population. Microplate enzymatic assay An sundaicus showed the resistance level ranging from 2,92 % to 31,25 % due to insensitive acetylcnolinesterase, except An. sundaicus collected from Cilacap Regency 33,33 % resistance due to elevated esterase activity mechanism. High esterase activity was found on An. aconitus and caused resistance level ranging from 8,8 % to 20,83 %, while An. aconite collected from Pekalongan Regency 6,7 % resistance due to insensitive acetylchoiinesterase.The implication of this research on malaria vectors collected from several Regency in Central Java and Yogyakarta Provinces showed an insensitive acetylcholinesterase earning resistance to both insecticides. Therefore the use of another insecticide group for vector control was suggested.