Resistance mechanisms of a strain (PRAIAS) of northern house mosquito, Cu-lex pipiens L., collected in Portugal in 1993, and highly resistant to organophosphates and carbamates, were investigated by comparing the resistance characteristics to 3 organophos-phorous (temephos, chlorpyrifos, malathion) and 1 carbamate (propoxur) insecticides in the presence or absence of synergists; and by determining the possible occurrence of overpro-duced esterases or insensitive acetylcholinesterase (AChE). The reference strain MSE from southern France, with an insensitive AChE, was included in all analyses for comparison. For organophosphorous insecticides, resistance in PRAIAS was caused by an insensitive AChE and an increase in oxidative metabolism. although the 2nd mechanism has only a marginal effect. For propoxur, the insensitive AChE was the only resistance mechanism detected. Biochemical properties of both the French and Portuguese insensitive AChEs were similar. We cannot exclude the possibility that PRAIAS and MSE strains possess exactly the same insensitive AChE allele.