Artificial selection for increased resistance to methidathion in two replicate lines of the whitefly, Bemisia tabaci (Gennadius) (originating from a cotton-field-collected greenhouse population) was successful; LC(50)s increased 7-8.6-fold in eight generations in the selected lines. This indicates the existence of additive genetic variance for resistance in the source population. Minimal realized heritability estimates, calculated from the response to selection, were h(2) = 0.49 after one generation (with minimal effects of common laboratory environment and inbreeding) and a mean value of h(2) = 0.344 after eight generations. Esterase activity (measured from the hydrolysis of beta-naphthyl butyrate) increased in the selected lines in correlation with resistance. We observed no change in mean esterase activity in the unselected (control) population. No consistent differences in fitness components between selected and control lines were detected during selection, but females exposed to sublethal doses of methidathion tended to have increased fecundity.