Larval gut characters of 45 species of black flies, representing 17 subgenera and 15 genera, were studied for their utility in taxon diagnosis and phylogenetic inference. Phasecontrast and scanning electron microscopy revealed a wealth of new characters in the esophageal armature of the foregut. Larval gut morphology, particularly characters of the esophageal armature, varied significantly among taxa, permitting generic diagnoses. A cladogram inferred from 13 gut characters supported a sister-group relationship between Parasimulium and the remaining simuliids, but did not show a strong phylogenetic signal for the current, well-supported concept of tribal level monophyly.Additional key words: esophageal armature, morphology, peritrophic matrix Invertebrate Biology 128(2): 182-204. r