An iridescent virus was isolated from larvae of the black fly, Simulium vittatum, collected from the South Saskatchewan River. Infected larvae displayed a brilliant blue-green iridescence. The virions occurred in paracrystalline arrays in the cytoplasm of fat body and epithelial cells. The apex-apex diameter of the icosahedral virions measured 127.7 ± 0.9 nm in negatively stained preparations. Characterization of virion structural polypeptides on SDS-polyacrylamide gels resolved 26 polypeptides ranging from 12.7 to 205.8 kDa. Analysis of EcoRI, HindIII, and PstI restriction endonuclease digests of purified viral DNA indicated that the genome had an approximate size of 153 kbp. On the basis of the restriction endonuclease profiles, this iridescent virus is a new isolate from black flies. © 1990.