Abstract : Epistemic cultures and scientist's public commitment in the GMO controversy. From the Berg letter in 1974 to the petitions “Défendons la recherche” and “Ouvrons la recherché” in 2003, biologists have often alerted –and been challenged in– the public space, concerning the risks and stakes of genetic engineering. The paper first compares the public commitment of scientists on GMOs in the 1970's and in the recent years. Around 1975, a short lasting GM controversy arose from the mobilization of young committed biologists. Sharply in contrast with this proactive commitment, most of the seven petitions signed by scientists after 1996 were rather reactive and provoked by GM field trials' destructions. These results are discussed in the wider context of the transformation of scientists' public commitment since 1968. Among the 3217 petitioners of these petitions, the paper then shows a correlation between disciplinary belonging and the public opinion expressed on GM crops. The paper finally shows how the trajectory of the French GM controversy in public arenas has co-evolved with the competition between three epistemic cultures (molecular biology, population biology and farming systems agronomy) for framing GMOs risks and biosafety.