There are 25 extant black fly (Simuliidae) genera, six of which build "wellformed" cocoons, including Austrosimulium, the only simuliid represented in New Zealand. The well-formed cocoon has been used in several analyses to suggest a close relationship among these six genera. A detailed account of cocoon-spinning behaviour would both provide a reliable indication of the homology of the well-formed cocoon structure, as well as indicate which behavioural features are important for the construction of a well-formed cocoon. Austrosimulium cocoon-spinning information was added to an existing behavioural phylogeny of the Simuliidae, which included two other genera with well-formed cocoons. One most parsimonious tree is produced (length 21, CI 0.95, RI 0.95), with the three genera constructing well-formed cocoons (Austrosimulium, Ectemnia and Simulium) resolved as a monophyletic group sharing six derived character states. Austrosimulium is resolved as the sister group of Simulium, the most derived and speciose genus of black fly, based on three synapomorphies.