Plants from two cultivars of forage rape and one cultivar of oilseed rape were inoculated with turnip root fly (Delia floralis) eggs at either 4 or 6 weeks prior to harvest, or, alternatively, were artificially damaged by mechanically removing approximately one-third of the root volume. Larval damage increased the total glucosinolate content of the roots. In contrast, artificial damage reduced the total glucosinolate content of the roots, with the levels decreasing as the time interval between damage and harvest lengthened. Turnip root fly larval damage also modified the glucosinolate composition of the roots, with the proportion of indoleglucosinolates increasing significantly. No such changes were observed in the artificially damaged roots. Neither type of damage appeared to affect the total glucosinolate content of foliar samples significantly, when averaged over all cultivars, but it was of interest to note that the proportion of glucobrassicin present in the leaves of the oilseed rape cultivar, Ariana, was consistently greater in both the larval and artificially root damaged plants compared with undamaged plants of the same cultivar. These results are discussed in relation to sulphur availability and the validity of root trimming as a simulation for turnip root fly damage.