Diaeretiella rapae (M'cintosh) was the only parasitoid of the cabbage aphid, Brevicoryne brassicae L., during studies reported here. Percentage parasitism of B. brassicae ranged from 27.1 to 76.1% in studies carried out around Nairobi area during 1978/79. It was highest during the month of September, when the aphid population density had reached maximum, and lowest in May when aphid numbers were low. High parasitism coincided with a decline in aphid population. Because of these high levels of parasitism, it was concluded that D. rapae played a significant role in suppressing populations of B. brassicae and should be taken into consideration in any control programme aimed at protecting brassica crops against aphid pests in Kenya. none