The cytochrome P450 dependant monooxygenases (P450s) are a vital biochemical system that metabolizes xenobiotics such as pesticides, drugs and plant toxins, and regulates the titers of endogenous compounds such as hormones, fatty acids and steroids. Through standard backcrossing procedures we were able to create a resistant strain of Culex pipiens quinquefasciatus isogenic to SLAB (a susceptible strain) containing only the P450 detoxification mechanism, which we named ISOP450. Resistance in ISOP450 is incompletely dominant (D=+0.3) and autosomally linked. Dose-response lines for the backcross progeny were not significantly different from the null hypothesis of monofactorial inheritance. The ISOP450 strain allowed us the unique opportunity to determine the level of cross-resistance conferred by the P450(s) to pyrethroid insecticides with various structures. Resistance ratios were dramatically lowered when either an α-cyano group was present or when the compound did not contain a 3-phenoxybenzyl moiety. This indicates that for ISOP450 the non-3-phenoxybenzyl alcohol and α-cyano groups are bulky enough to physically block the site of metabolism, hindering the detoxifying action of the P450 that confers resistance. The level of resistance that this P450 provides to other classes of insecticides will also be presented.