Current recommendations to delay the evolution of resistance to Bacillus thuringiensis crops are that a minimum of 5-50% of a crop-growing region should include non-B. thuringiensis varieties as refuges. These recommendations are based in part on the assumption that resistance will be inherited as a recessive trait. Laboratory microcosm experiments are described with transgenic canola expressing Cry1Ac and a non-recessive Cry1Ac resistant population of the diamondback moth Plutella xylostella, in which the effect of different sizes of refugia (0%, 20% and 50%) on resistance was compared over five generations. The LC50 values for Cry1Ac increased markedly in the P. xylostella sub-populations with 0% (>100-fold) and 20% refugia (>35-fold) but showed little change (less than fivefold increase) with 50% refugia. The results support the idea that relatively high levels of refugia (non-B. thuringiensis varieties) may be required where resistance is not functionally recessive at the level of toxin expressed in the B. thuringiensis crop.