A new monotypic genus, Aztecamyia
gen. nov., is proposed for a new species of muscid fly, Aztecamyia tlaloc
sp. nov., found in highland localities from the Mexican Transition Zone (Mexico, Michoacan, Morelos and Puebla). Additionally, a morphology-based phylogenetic analysis of Coenosiini is presented. The new genus has some unique characters, such as katepisternal setae 0+1+1 in males, while females are the typically Coenosiini 1+1+1; the mid and hind tibia have many long and fine setae on the apical half, from the anterior to posterior surface; and the preapical setae on all femora are absent on all surfaces. The morphology of the male and female terminalia is a typical Coenosiini shape.