<p> The relationship between the mixed-function oxidase (MFO) system and resistance of Drosophila melanoqaster to phosmet (ImidanR) an organophosphorus insecticide has been studied. The MFO activity was assayed by 7-ethoxycoumarin-O-deethylase (7-ECD) activity and resistance levels were determined by either adult LC- 50 or percent adult survival at 150 uM of Imidan. Two groups of isofemale lines sampled from Vineland, Ontario and a laboratory selected population were used to study variability of the MFO system and its relationship to variability in resistance. </p> <p> A significant positive correlation was found between 7-ECD specific activity and an antigenic determinant assayed by western blotting. The antiserum used to detect this determinant was prepared against a cytochrome P-450-enriched fraction from a malathion-resistant Drosophila melanoqaster strain by J. Pursey (1989). The antigenic determinant was contained on polypeptide P2, believed to be a cytochrome P-450 by its molecular weight and heme content. </p> <p> Laboratory selection on Imidan resulted in over 2 .1-fold increase in resistance. Two MFO activity groups were identified among isofemale lines established from nature. A siqnificant positive correlation was found between 7-ECD specific activity and resistance to Imidan among the Isofemale lines. The minimum number of genes controlling the difference in 7-ECD specific activity between two isofemale lines was estimated to be about 1. </p> <p> It was concluded that this natural Drosophila melanoqaster population has a major or few .genes on chromosome 3 affecting MFO activity and resistance but as well other factors on chromosomes 1 and 2 also contribute to variability in resistance. </p> / Thesis / Master of Science (MSc)