Genetically modified organisms (GMOs) have caused considerable controversy in China in recent years. Uncertainty about the technology, ineffective channels for releasing official information and a lack of sufficient public trust in the government and scientists have led to rampant rumours about genetic modification technology, making it hard for the public to acquire scientific knowledge about it and a rational attitude towards it. In this paper, by using as an example the rumour that genetically modified (GM) soybeans cause cancer, we discuss the content and diffusion of rumours related to genetic modification technology in the new media environment. Based on an analysis of content on the social media platform Weibo one week after the rumour began, we discovered that the ensuing cyber discussions reflected reality, that netizens expressed anxiety and panic while stressing social injustice and reflecting conflict between social classes, and that they exhibited little trust in scientists and the government. On the mechanism of diffusion of rumours on Weibo, we observed that ‘evidence’ that directly or indirectly purported to show that GM soybeans cause cancer was added to the rumours and that the rumours were ‘assimilated’ into people's perception through the stigmatization of GMOs and through conspiracy theories.