This article takes as its case study the “GM Nation?” public debate, a major participation process on the commercialization of agricultural biotechnology, which occurred in Britain during the summer of 2003. We investigate possible self-selection biases in over 36,000 open questionnaire responses on the risks and benefits of genetically modified crops and food obtained during GM Nation? A comparison sample of equivalent responses from a statistically representative sample (n= 1,363) of the British general public obtained shortly after the conclusion of the debate is reported. This comparison shows that the GM Nation? open responses were indeed not fully representative of British “public opinion” regarding agricultural biotechnology. Rather, such opinion is not a unitary whole, but fragmented, with considerable ambivalence coexisting alongside outright opposition to GM agriculture. The methodological implications for multistage participation processes are discussed: in particular, the need to anticipate outcomes of complex design decisions, and to include representative public surveys as standard where measures of broader public attitudes to risk are an important objective.