none Estimates of parasitism in samples of the cabbage aphid, Brevicoryne brassicae (L.) by the braconid Diaeretiella rapae (McIntosh) in a field population in Massachusetts (USA) on collards were 2.3-4.3 times greater for "scattered" aphids than for aphids in colonies (defined as five or more aphids in physical contact). Laboratory and field experiments were performed to see if aphids separated from colonies were subject to higher rates of successful parasitoid attack, or if parasitized aphids were more likely to become separated from colonies because of increased rates of movement over the plant. Parasitism levels of "scattered" aphids on potted collard plants exposed for 3 days in a collard field were 2.3-3.2 times higher than parasitism levels for aphids in colonies on identical potted plants at the same location for the same period. In laboratory experiments, aphids in colonies subjected to parasitism showed more movement than controls at 1 hr and 4 days after parasitoid attack, but at 8 days after attack cumulative amounts of movement of aphids in colonies subject to parasitism and control colonies not subject to parasitism were equal. From these results we conclude that higher levels of parasitism in "scattered" cabbage aphids seen on field plants are likely the result of the combined effects of higher levels of successful parasite oviposition in aphids already outside of colonies and higher rates of movement of aphids away from colonies after colonies are attacked by parasitoids.