To better understand the evolutionary radiation of the Simulium arcticum complex of black flies (Diptera: Simuliidae), we compared the geographic distributions of present-day larvae to their sex-chromosome diversity. We used the 5 known data sets including collections and sex-chromosome analysis from 307 geographic locations of 31 taxa of approximately 20,000 larvae from throughout the geographic range of distribution of the complex, from Alaska, western Canada, and the western United States to southern California, Arizona, and New Mexico. Siblings (reproductively isolated in sympatry) have considerably larger geographic distributions than do cytotypes (not reproductively isolated in sympatry), suggesting that the former may have been in existence longer than the latter. Simulium negativum (the oldest member of the complex), S. brevicercum (standard noninverted sex chromosomes), S. saxosum (sex determination on the X chromosome), and S. arcticum s. s. (IIL-3) share geographic distributions with all other siblings. Notably, 21 of 22 cytotypes share geographic distributions within those of siblings. Cytotypes are almost always discovered within the geographic distributions of siblings, suggesting that the former might be arising sympatrically.