In the transmission of human onchocerciasis, competence of the black fly vector influences intensity of transmission by regulating how many infective-stage larvae are produced. The disease is intensifying in Ecuador where Simulium exiguum is a highly efficient limitation-type vector comparable with African forest species in terms of the percentage of flies developing infective-stage larvae and the numbers of larvae per infected fly. In contrast, the endemic foci in Guatemala and southern Mexico have remained stable and closely circumscribed around larval development sites for S. ochraceum, a vector that exhibits a facilitation type of competence and must feed on people with comparatively high microfiladeremia to develop significant numbers of infective-stage larvae. This comparison indicates that assessments of vector competence should be included when planning regional control programs for onchocerciasis.