Laboratory colonies of the black fly Simulium vittatum Zetterstedt cytospecies IS-7 were analyzed cytogenetically and compared with the original New York population from which source material was collected 18 yr earlier. All sex chromosomes and major autosomal polymorphisms that were present in the source population were still represented in the laboratory colonies. However, the extent of sex linkage and the frequencies of four of the five major autosomal inversions changed significantly in at least one colony, perhaps because of bottlenecks experienced by the colony. A complete absence of males homozygous for the IS-7 inversion in both field and colony material is explained by postulating that the Y₂ chromosome, representing the inverted condition for the IS-7 sequence, is absent from the population or acts as a rare reproductive lethal. This example possibly represents pseudo-partial sex linkage involving the X-linked sequences.