Adults of Costelytra zealandica (White), treated by topical application of DDT in olive oil solution, showed differences in susceptibility between two collections taken from untreated paddocks and one from a paddock that had received applications of DDT, the latter containing 60–70 per cent of individuals surviving dosages that produced complete mortality in the first two. In the susceptible collections, males were two-three times as susceptible as females, at the LC50 level; in the tolerant one males were also more susceptible than females. In males, the LC50 of the susceptible samples was about one-sixth that of the most tolerant fraction of the tolerant sample. The difference in live weight between the sexes was insufficient to account for their difference in susceptibility; the dry weight of females was nearly double that of males, however, although there was little difference in their fat content. Observations on the seasonal increase in weight and susceptibility to DDT and fenthion in samples of larvae of C. zealandica, from four areas of different fertility in the same paddock, showed a general correlation between weight and susceptibility, masked in the case of DDT by great fluctuations in the latter. The mean weight of samples of female larvae from several field populations. was higher than that of the corresponding males, in some cases significantly so, but the difference was insufficient to cause differential susceptibility to DDT between the sexes in laboratory tests. However, an inverse correlation between weight and susceptibility was established among groups of larvae standardised by weight. It is concluded that, in studies of relative susceptibility to insecticides, especially DDT, in larval populations of C. zealandica, standardisation of test batches by weightwill materially decrease heterogeneity, but separation of the sexes is unlikely to do so. It is also concluded that adults, which give a clearly distinguishable mortality response, are useful test subjects in investigations of the composition of susceptible and tolerant populations.