Field studies on responses of two mosquito sibling species, Anopheles arabiensis Patton and An. quadriannulatus Theobald, to a man, a calf and different release rates of carbon dioxide (man, calf and cow equivalents) were conducted in north-eastern South Africa. Various combinations of baits were compared in two-choice tests, using two mosquito nets, placed 2.5 m apart and 10 cm off the ground. Mosquitoes attracted to the baits were able to enter the nets from below and were collected by means of a suction tube. In a two-choice test between a man and CO2 (human equivalent, 250 ml/min), 81% of the An. quadriannulatus were caught with CO2. The reverse was seen for An. arabiensis, where only 20% of the total catch was caught with CO2 compared to man. High release rates of CO2 (cow equivalent, 800 ml/min) attracted significantly more An. quadriannulatus than the low release rate (250 ml/min), whereas no significant effect of the release rate of CO2 on the total catch of An. arabiensis was seen. In the latter species, up to 33% of the attraction of human emanation is attributable to carbon dioxide. Anopheles quadriannulatus was equally attracted to a calf and CO2 (calf equivalent, 180 ml/min). Catches of other mosquito species showed consistent differences between all treatments which appear to be associated with differences in host-preference, suggesting that the importance of CO2 in host-seeking behaviour of mosquitoes increases with the degree of zoophily.