Abstract: It was found by study of biochemical properties of acetylcholinesterase (RR-and SS-AChE) that the propoxur-resistant (RR) and susceptible (SS)-AChe of the housefly,Musca domestica were different in the following aspects. 1) The optimum temperature of AChE in the RR and SS strains was found at 37℃ and 34℃,respectively.But the optimum pH value was all the same,7.4. 2) The capability of substrate hydrolyzed by the two AChEs existed markedly difference.The V/em>max ratios of RR-to SS-AChE were 2.22 for ATCh,1.08 for BTCh and 3.41for PTCh. 3) Based on the bimolecular rate constant (Ki), the RR-AChE was 46.77-,28.15-,66.15- and 15.00-fold less sensitive to four carbamates propoxur,carbofuran,carbaryl and methomyl and was 7.66-,12.13-,3.81 and 2.25-fold less sensitive to four organophosphates malaoxon,methamidophos,omethoate and triazophos oxon,respectively. 4)The affinity constant (Ka) values of the tested inhibitors with RR-AChE were all higher than that of SS-AChE. 5) The phosphorylation or carbamylation constant (K2) values of RR-AChE were all less than that of SS-AChE.All these data implied that RR-AChE was qualitatively altered.