In both experimental and naturally developed colonies, Aphis fabae Scop., and Acyrthosiphon pisum (Harris) were concentrated on the upper parts of broad bean plants. A. pisum resembled Myzus persicae (Sulz.) rather than Aphis fabae by normally infesting leaves, not stems. Colonies of M. persicae were dispersed over the whole plant. In laboratory experiments, apterous adult aphids were observed walking on bean plants, and their activity was recorded in terms of their actions at each node. Moving aphids of both species tended to maintain their direction at a node although the structure of the plant does not appear to favour this. Aphids of both species changed their direction of movement more often after going onto the leaf at a node than when they stayed on the stem. M. persicae adults went upwards as often as they went down, but those of Acyrthosiphon pisum walked up more often than down and were more likely to turn round if they were moving downwards. These differences in the behaviour of active individuals would tend to produce the differences between the observed spatial distributions of the two species, although other factors, including leaf age and reproductive rates are probably more important in determining the form of aphid colonies.