none The numerical aspects of interactions between Diaeretiella rapae and its host aphid Lipaphis erysimi bred on three different brassicaceous plants at different parasitoid densities were studied. When one, two, four or eight parasitoids were released on 200 of each aphid/host plant combination, the number of emergents showed a significant positive correlation with increasing parasitoids on all the tested food plants. The rate of multiplication, expressed as parasitoids egressed/female parasitoids put in, was greatest with one parasitoid for 200 hosts. It decreased with an increasing parasitoid number and also in the sequence of Brassica oleracea (cv. Pride of India), Brassica campestris (cv. Varuna) and Raphanus sativus (cv. Japani white). The fact that more parasitoids have a lower rate of multiplication shows the existence of mutual interference. Various aspects of the behaviour of the parasitoid which could give rise to interference effects are discussed. The killing power (k-value) of the parasitoid increased significantly with an increase of parasitoid density on all the tested food plants.