Neuroblast and salivary gland chromosomes were studied in Feulgen squash preparations of Simulium vittatum larvae. The haploid complement consists of three metacentric chromosomes; the arm ratio of the longest (1) and shortest (3) is approximately as 1: 1, that of the second as 1: 2. Centromere regions of salivary gland chromosomes are expressed as characteristic expanded drums with large disorganized chromomeres. The third salivary gland chromosome bears the single large nucleolus and a smaller granular structure comparable to the Ring of Balbiani of chironomids. Pairing of the two constituents of salivary gland chromosomes is quite loose regardless of the degree of polyteny. Photographic maps are given of the standard band sequence of the entire complement. Relatively short inversions are the commonest type of structural rearrangement. From preliminary observations on further black fly species and an eclectic treatment of the nematocerous literature, morphological characteristics of salivary gland chromosomes are discussed with respect to their value in delimiting taxonomic segregates and determining phylogenetic relationships.