Abstract: Many researchers have shown that the numbers of pest insects found on cruciferous and other crop plants are reduced considerably when they are grown with other plant species. The aim of a recent project at University of Warwick was to use companion plants instead of insecticides for controlling the cabbage root fly in cauliflower crops. This paper describes some initial experiments to determine how the height, leaf area, leaf shape, proximity and spatial arrangement of companion plants affects host plant selection and oviposition by female cabbage root flies on cauliflower plants. Artificial companion plants were used in the experiments, so that the effects of companion plant architecture could be evaluated critically. The results indicated that to be most effective, the companion plants should be in close proximity to the cauliflower plant. There was evidence that the height of the companion plant foliage was important and that plants that were at least half as tall as the cauliflower plant were more effective, indicating that, to be effective, companion plants have to be relatively tall and erect at the time when the cauliflower plant is most susceptible to damage by cabbage root fly larvae. The size and shape of the leaves of the companion plants appeared not to be critical, allowing some flexibility in the choice of companion species.