In the risk debate over genetically modified (GM) crops, Europe's regulatory delays have often been branded as 'political', i.e. not based on science.  Yet the US slogan 'sound science' tends to conceal value-laden features of safety claims, their weak scientific basis, their normative framing and their socio-political influences.  By contrast a 'precautionary approach' can more readily identify scientific unknowns to be investigated, while acknowledging the agricultural-environmental values which inform risk assessment.  These issues underlie transatlantic regulatory disputes over insect-protected Bt maize.  In both the USA and Europe, public protest has stimulated risk-assessment research on broader cause-effect pathways, as well as more stringent regulation.  For harm to non-target insects, however, new evidence of risk has been disparaged as unsound.  It has been criticized on various grounds, which could apply just as well to evidence of safety; thus double standards have served to protect safety claims.  And non-target harm is deemed acceptable through unsubstantiated comparisons to agrochemical usage. In these ways, 'sound science' operates as an ideology, pre-empting debate on the framing of scientific uncertainty.  The real choice is not between 'science versus politics', but rather between ways of linking them.