['Life history parameters are useful tools for comparing the fitness of pests on different host plants. This study compared life history parameters of twospotted spider mites (Tetranychus urticae Koch) on two resistant cotton Gossypium genotypes (BM13H and Sipima 280) and one susceptible genotype (Sicot 71). The effects of both constitutive and induced defences were assessed.', 'Mites reared on the resistant genotypes had longer immature development times, lower immature survival and reduced adult fecundity. Mites reared on BM13H that had been induced by prior exposure to mites had a small additional decrease in adult fecundity. The contribution to mite resistance of constitutive resistance mechanisms was much greater than induced responses. The effect of morphological constitutive defences was minor, implicating biochemical defences as the major mite-resistance mechanism. Sensitivity analysis and a population development study using life history parameters of mites showed that a lower immature survival rate on resistant genotypes had the greatest effect on mite fitness and population development.'] Is fitness of twospotted spider mites more affected by constitutive than induced resistance traits in cotton ( Gossypium spp . )?