Aggregation behavior of two aphids, Brevicoryne brassicae and Myzus persicae, coexisting on cruciferous plants such as cabbage was studied from a comparative viewpoint. The former formed compact aggregations and the latter "spaced-out" aggregations with 1-2 mm between the nearest neighbors. Newborn nymphs of B. brassicae had a strong tendency for contact with their mother, and nymphs of every instar and adult were gregarious, while newborn nymphs of M. persicae tended to settle at a distance of about 2 mm from the mother. Most of the B. brassicae retained contact even at low densities, while few M. persicae were in contact with each other even at high densities, The effects of aggregations of the two aphids on their population dynamics were discussed.