The compatibility between sympatric and allopatric combinations of Onchocerca volvulus–anthropophilic species of Simulium was studied in the north-eastern focus of human onchocerciasis as well as in a densely populated locality of the Amazonas State in Venezuela. The objectives were to test the conjecture that local adaptation exists between the parasite and its vectors (the Onchocerca–Simulium complex hypothesis), and assess the possibility of the infection spreading from its present distributional range. For the homologous combination, O. volvulus–S. metallicum cytospecies E in Anzoátegui State (north-eastern focus), parasite yield was 45% in contrast to 1% for the heterologous, southern parasite–S. metallicum infection. This was significantly lower than the parasite yield (4–10%) expected after allowing for the effect of density-dependent limitation of infective larval output described in this paper for S. metallicum. The population of S. exiguum s.l. from southern Venezuela allowed no larval development beyond the L1 stage of either northern or southern parasites. Mechanisms for such refractoriness probably operate at the level of the thoracic muscles, not affecting microfilarial uptake or migration out of the bloodmeal. The parasite yield of southern O. volvulus in S. oyapockense s.l. flies biting man at Puerto Ayacucho (Amazonas) was about 1%, in agreement with the figures recorded for highly compatible sympatric combinations such as O. volvulus–S. ochraceum s.l. in Guatemala. No infective larval development of the northern parasite was observed in southern S. oyapockense. These results, together with considerations of typical worm burdens in the human host, presence/absence of armed cibaria in the simuliids, parasite-induced vector mortality, and fly biting rates, suggest a lower potential for onchocerciasis to spread between the northern and southern endemic areas of Venezuela than that between Amazonian hyperendemic locations and settlements outside this focus with high densities of S. oyapockense s.l.