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 degrees 41' latitude in Florida to 44 degrees 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.