Olfactometer experiments were conducted with apterae and alatae of the cowpea aphid, Aphis craccivora, Koch (Hom.:Aphidoidea). The occurrence of density dependent related pheromones and odour response to their host plant Vigna unguiculata was studied. Apterae responded with positive anemotaxis to air passed over both small groups of apterae and alatae (≤10 individuals) but negatively to air that passed over a bigger group (≥20 individuals). Alatae responded in the same way to groups of apterae but were repelled by alatae independent of the size of the group applied as odour source in the olfactometer, except when exposed to air passing ten alatae feeding on cowpea leaves. This may be due to interaction between aphid pheromones and host plant odour. Both apterae and alatae showed a general attraction to green plants and were also able to distinguish between other host plants and cowpea, which was the original host of the aphid clone used in the experiments. When attacked by aphids, the cowpea plant responded with a temporal increase in attractivity that reached a maximum after 48 h and had disappeared after one week.