The karyotype of Haematobia irritans is reported along with data on nucleolar organizer regions (NORs). The karyotype (2n=10) is characterized by the presence of 3 metacentric and 2 submetacentric pairs and the absence of heteromorphic chromosomes. One of the submetacentric pairs seems to present a pericentromeric NOR. Among the Calyptrate, the horn fly is one of the few species that does not conform to the general rule of 2n=12.