Populations of Plutella xylostella L. with high (laboratory re-selected) and low levels (unselected) of resistance to the Bacillus thuringiensis (B.t.) toxin Cry1Ac were maintained under controlled environment conditions in the absence of any further selection pressure. Larvae were maintained either in discrete generations on freshly excised leaves replaced ad lib (standard culture system) or in microcosm cages holding up to 12 potted Chinese cabbage plants, allowing progressively overlapping generations of insects. After 12 generations in the microcosm cages, the LC50 values of both selected and unselected populations were not significantly different from those of a standard susceptible population. Under standard culture conditions, the LC50 value for unselected insects after 12 generations was almost identical to that of unselected insects in the microcosm cages, whereas the LC50 value for selected insects was 1250-fold greater. The fecundity of the selected population was significantly lower compared with that of the same population in the microcosm cages after 12 generations. There was no significant change in the fecundity of the unselected population after 12 generations in the microcosm cages. The results show, that differences in the culture system (microcosm vs. standard) and in the resistance level (re-selected vs. unselected) can have a marked influence on the stability of resistance to a B.t. toxin in an insect population. Possible factors involved in determining the rate at which resistance declines are discussed.