This study summarizes cytogenetic variation, particularly sex-linked chromosomal inversions which define taxa of the Simulium arcticum complex (Diptera: Simuliidae) in western Montana and portions of northern Idaho, Washington, and Oregon. Variation in polytene chromosomes was determined for nearly 15,000 larvae from 234 collections taken from 58 freshwater sites. The previously described siblings, S. apricarium, S. arcticum sensu stricto, S. brevicercum, and S. saxosum, were most numerous, while all cytotypes, with the exception of IIL-19, were found in low frequency. Additionaly, 6 new cytotypes in low frequency are described. Evidence suggests that the Y chromosome carries the testis-determining gene, and in almost all taxa of S. arcticum, complex paracentric inversions characterize these types. Distributions of (1) S. brevicercum and S. arcticum s. s., (2) S. arcticum s. s. and S. arcticum IIL-18, (3) S. brevicercum and S. arcticum IIL-18, and (4) S. arcticum IIL-9 and S. arcticum IIL-19 have highly significant positive geographic associations, while those of (1) S. apricarium and S. brevicercum and (2) S. apricarium and S. saxosum have highly significant negative geographic associations. The S. arcticum cytotypes IIS-12, IIL-17, IIL-21, IIL-22, IIL-38, IIL-51, IIL-68, IIL-73•74, and IIL-79 occur only at 2 or fewer locations. Polyploids (0.0007), pericentric inversions (0.00007) and chromosomal translocations (0.00007) are exceedingly rare. These observations and our recent DNA comparisons of chromosomally distinct types lead me to elaborate on a previously suggested model for chromosome evolution in black flies. In this model, locally distributed cytotypes may become more common with time, while widespread cytospecies may eventually become morphologically differentiated types. Contrary to the current understanding that chromosome variation may not play a significant role in the speciation process of most animals, this study suggests that chromosomal variation, at least in black flies, plays a significant role in speciation.