The influence of cabbage root fly [Delia radicum (L.)] on cauliflower dry matter production was investigated in three experiments during 1992 and 1993. Cauliflower plants grown outdoors in pots were inoculated with different numbers of cabbage root fly eggs, 1, 2, 3 or 4 weeks after transplanting as seedlings. A non-destructive method was used to estimate dry matter production during the growing season. Regression analyses of plant dry matter against egg number demonstrated that control measures against cabbage root fly larvae are needed to protect young seedlings during the first 3-4 weeks after planting, whereas 100 eggs or more were required to cause a 5% reduction in mean plant dry matter when plants were inoculated 4 weeks after transplanting. Egg and larval mortality varied with means in the range from 47 to 61% and density-dependent larval mortality was demonstrated in two out of four combinations of experiment and inoculation time.