Some adult female black flies (Diptera: Simuliidae) bite humans and can be pests in high numbers. In areas of high black fly abundance, larvicide-based suppression programs reduce the emergence of adults from rivers and streams. However, little is known about whether either of the sexes develops or emerges first. Descriptions of sexual development might help control managers decide when application of pesticides is most effective. For the past 14 yr the cytogenetic diversity within the Simulium arcticum (Malloch) complex of black flies in the Pacific Northwest has been studied and described. These descriptions have been accompanied by an “on-slide” identification of the gonads and, thus, the sex of individual larvae. Consequently, a very large data set is available to describe sex ratios during larval development and determine if either sex develops before the other. The sex of >11,000 larvae from 161 collections from 41 geographic locations has been monitored. Larvae having white histoblasts were chosen for chromosome studies, but no selection of either sex was made. Males outnumbered females in first collections at almost all sites and those made before 31 March of any year. Between 31 March and 15 April, females slightly outnumbered males, but after 15 April the ratio of the sexes was similar. These data also suggest that this approach might be inexpensive and useful in the immediate determination of sex ratios because an estimate of the ratios could be made within hours of application of larvicides.