We made 37 collections and analysed the polytene chromosomes of salivary glands of 726 larvae of the Simulium arcticum Malloch (Diptera: Simuliidae) complex from 10 locations in an unstudied region from central Idaho and southeastern Washington, United States of America. We compared our results to previous population cytotaxonomic research on larvae of this complex from western Montana, northern Idaho, and eastern Washington, United States of America. We identified four sibling species, S. brevicercum Knowlton and Rowe, S. saxosum Adler, S. arcticum sensu stricto, S. apricarium Adler, Currie, and Wood; and three cytotypes, S. arcticum IIL-9, IIL-17, and IIL-79, previously described by us. We discovered a new cytotype, S. arcticum IIL-80, at three locations in the western region of our sample area. We also found combinational (ancestral) types between S. saxosum and S. arcticum sensu stricto and between S. saxosum and S. arcticum IIL-79, suggesting that ancestral populations of the complex still exist. Geographic structuring of these sibling species and cytotypes are documented given that S. saxosum occurred in western regions, S. arcticum IIL-79 in northeastern regions, and S. apricarium in southeastern regions of our study area.