Field and laboratory studies support recent hypotheses that Sirystes sibilator is closely allied with the myiarchine flycatchers rather than with Tyrannus or with the cotingids. The first confirmed nest of this tyrant (located in a natural tree cavity), its foraging behavior, and the derived character states of the nasal septum and of the syrinx argue convincingly for its placement in an assemblage with Myiarchus, Rhytipterna, and Casiornis. Data, still lacking, on the color of the eggs and nature of the nest lining are needed for Sirystes (and for Rhytipterna and Casiornis) before consideration can be given to the generic relationships within the myiarchine flycatchers.