Geographic variation in the chromosomal location of the male sex determining factor (M) was studied in four house fly, Musca domestica L., populations from the eastern United States. We found a strong clinal trend (29° 41′ latitude in Florida to 44° 2′ in Maine) in which the percentage of standard XY^M males increased with increasing latitude. In Florida, 100% of the males possessed the M factor on the third autosome (III^M). North Carolina had 20% III^M males and 2.35% with both Y^M and III^M. Fewer III^M males were found in New York (4.35%). Populations from Maine contained 100% XY^M males. In two of three standard laboratory-susceptible strains, all males carried M on an autosome (“autosomal males” or A^M): CS (III^M) and SRS (V^M). Insecticide bioassays of four field-collected strains led us to conclude that resistance is not correlated with sex determination over a broad range of insecticides. For example, high levels of resistance to permethrin (86–99% survival at a diagnostic concentration) were found in all four field-collected strains. The five other insecticides evaluated showed varying levels of resistance among field strains. We conclude that a cline is present in house fly populations from the eastern United States with 100% III^M males in the south and entirely Y^M males in the north and that insecticide resistance is not a key factor influencing the evolution or linkage of M.