I made six extensive collections of black fly larvae at the Little Blackfoot River (LBFR) in western Montana at roughly 2-week intervals from 25 March to 2 June 2019 to describe larval size-class frequencies and cytogenetic diversity of the first spring generation. Frequencies of seven size classes, pupae, and pupal exuviae are given, though no larvae with egg bursters (first-instar larvae) were observed. Therefore, the earliest instars were missed. Sex chromosomes of 562 larvae of this first generation were compared to those of 1260 other larvae previously analyzed from 10 dates (2003 to 2012) to determine whether sex chromosome frequencies differed over time. The fifth and sixth size classes, those having white gill histoblasts, possess the best polytene chromosomes for analysis. The LBFR is a very diverse site having four sibling species and nine cytotypes present. However, the sibling species Simulium brevicercum (IIL-st/st) and S. arcticum s. s. (IIL-3 st/i) are most abundant, comprising 64.6% of the 2019 samples and 62.0% of the yearly samples. Simulium arcticum s. s. is abundant in early spring (2019), while S. brevicercum is rare. As the 2019 spring progresses, however, S. brevicercum becomes abundant, while S. arcticum s. s. becomes less frequent. This trend is not seen in the analysis of yearly samples, suggesting that single collections and analyses may not be accurate. In both the 2019 and yearly analyses, male larvae predominate early while female larvae eventually predominate in late spring, although only slightly. All taxa except S. brevicercum and S. arcticum IIL-10 are not significantly different from year to year. The proportions of these two taxa for the year 2019 were close to the overall average of both types; thus, 2019 was not an anomalous year.