I analyzed 221 black fly larvae of the Simulium arcticum complex from the Logan River, Logan Canyon, Utah, for larval sexual dimorphism, chromosomal inversions, morphology of the centromere band of chromosome IIL, presence of B chromosomes, and frequency of the IS-1 autosomal polymorphism. All 2275 larvae collected at the Logan River were of the S. arcticum complex with one exception, which was S. canadense. I found a significant association between larval color and sex, but I did not find such an association between larval head patterns and sex. Nearly all chromosomally analyzed larvae had standard, noninverted IIL chromosomes in both sexes. However, 70 of the 117 noninverted IIL chromosome males had the enhanced/thin (Ce Ct) centromeric dimorphism. Thus, there are at least two Y chromosomes in the Logan River population (Y₀ and Y₁). I also found four females and three males that had different chromosomal inversions at the base of chromosome IIL, but each of these was a singleton. Of the 120 males analyzed, six larvae had one acrocentric B chromosome and one larva had two acrocentric B chromosomes. None of the larvae at the Logan River was polymorphic for the IS-1 autosomal inversion. The presence of Ce Ct individuals that are also st/st for the IIL arm suggests that some of these larvae at the Logan River may be a new taxon. This observation is significant since some populations defined as Simulium brevicercum have only the IIL-st/st, Ce Ce genotype, while others have IIL-st/st larvae that have both Ce Ce and Ce Ct centromeric bands.