Males of the Beffa form of Simulium soubrense Vajime and Dunbar were successfully crossed with female S.squamosum V. & D. and S.sanctipauli V. & D. Fertile eggs from both crosses were reared to larvae and, in the case of the Beffa x squamosum cross, to adults. Larval chromosomes from the Beffa x sanctipauli cross had normally paired homologues but those of the Beffa x squamosum cross showed extensive asynapsis and large heterozygous loops. The morphology of the larval and adult hybrids from the Beffa x squamosum cross were more characteristic of squamosum, but the adult males had scutal patterns typical of the Beffa form and retained this form's laboratory-mating propensity.