Despite an extensive literature on mosquitoes, remarkably little attention has been paid to males. Current interest in control by release of transgenic males begs attention to this bias. It is well known that males are more susceptible to insecticides than females when determined by the standard World Health Organization (WHO) bioassay, and field observations have shown a higher impact of ultra-low-volume (ULV) space sprays. It is generally assumed that these differences are due to the smaller size of males and/or greater physiological susceptibility. We compared susceptibility by WHO bioassay and by topical application. There was a significant difference between the sexes in terms of dose effect and knockdown by the WHO test, but no significant difference by weight-adjusted topical application. We conclude that greater susceptibility of males is solely a function of their size and suggest that a ULV treatment before the release of transgenic males would greatly increase their competitive ratio versus wild mosquitoes and thus their impact as a control measure.