Aggregation, shown by both nymphal and adult lime aphids, is not due to mutual tendency to select particular feeding sites, but is due to social aggregation, the aphids themselves being the attractive stimulus. In adults aggregation comes about through responses to visual stimuli from the wing patterns of other adults. Antennal contact between adults does not appear to be important in inducing settling, indicating that possibly visual or olfactory stimuli may be involved. With increased density closer spacing takes place resulting in increased antennal contact between adults and the possible significance of this is discussed. Closer spacing is not simply a result of limited space available at high densities but is self imposed by the adults themselves.