The larval polytene chromosomes of 17 of the 22 known cytotypes of the Simulium arcticum Malloch complex were analyzed and compared at 59 collection sites in the Pacific Northwest of the United States. Cytotypes range from those that were broadly distributed, such as IIL-9, which was found at 25 different sites, to those that were found at only one or two sites, such as IIL-73·74. Cytotypes are “clustered” in a geographic sense, suggesting that once a new cytotype is described, if it is found again, it will be found nearby. The cytotype S. arcticum IIL-9 was found at 11 of the 25 sites where S. arcticum IIL-19 exists. Finally, based on its sibling species status and widespread distribution throughout the Pacific Northwest, Simulium brevicercum may be an early ancestor of the complex given that it occurs at 44 of the 59 sites sampled. The observations and conclusions of this research are scientifically important because previous research suggests that reproductive isolation may have begun among these cytotypes at these sites, and unless their locations are reported, they cannot be studied further. Future work should incorporate a multiomic approach to study the putative initial divergences within this complex.