There have been several major cases in the international debate on the biosafety of genetically modified (GM) crops, including the following. (i) The case of Pusztai's claim in 1998 that GM potato with inserted gna gene from snowdrop caused adverse effects on rat health. The peer review led by the UK Royal Society concluded that Pusztai's results were incorrect in many aspects and no such adverse effects should be inferred from them. (ii) In 1999, Nature published a paper by a group from Cornell University in which it was reported that the larvae of monarch butterfly were killed by the pollen of Bt corn on milkweeds. Subsequent laboratory and field studies have shown that pollen of Bt corn does not harm the monarch butterfly. The decline in population density of the monarch butterfly is caused mainly by the overuse of pesticides and environmental changes occurring in Mexico. (iii) In 1998, volunteer canola resistant to three herbicides was reported in a canola producer's field in northern Alberta, Canada, which was then called a ‘super weed’ by activists. In fact, ‘super weed’ is not a scientific term and no such case exists in nature: the triple-resistant volunteer canola can be killed by spraying with the herbicide 2,4-dichlorophenoxyacetic acid. (iv) In November 2001, Quist and Chapela published a paper in Nature claiming that DNA sequences similar to the CaMV35S promoter and adh1 gene used in GM corn Bt 11 were found in samples of maize landraces collected from Oaxaca, Mexico. Subsequent scientific analyses demonstrated that the sequence of the 35S promoter detected was an artefact and the sequence of adh1 was adh1-F, a native gene in maize and not the adh1-S transgene used in Bt 11. (v) In June 2002, Greenpeace published a report which stated ‘Bt cotton damaged the environment in China’. The positive benefits of Bt cotton in China were not cited in Greenpeace's report. The fact is that, as a result of commercialization of Bt cotton, the amount of pesticides used for cotton bollworm control has been dramatically reduced by 70–80%. Therefore, the population size of predators and the diversity of arthropods in Bt cotton fields have increased drastically, which resulted in a dramatic reduction of the aphid population by 443- to 1546-fold in Bt cotton fields compared with non-Bt cotton fields. Monitoring bollworm populations nationwide in cotton-growing areas has shown that none of them has developed resistance to the Bt protein or to Bt cotton to date. The migration behaviour of the bollworm, the inheritance of insect resistance to Bt controlled by an incomplete recessive gene, the existence of ‘natural refugia’ in multiple-cropping systems in North China and the use of transgenic cotton with double genes (Bt/CpTI) have all played important roles in delaying the development of resistance to Bt cotton in bollworm populations. In conclusion, international debates on the biosafety of GM crops are not purely a scientific issue, but are related to economic and trade considerations.