Oxynils which include bromoxynil and ioxynil are a family of antidicot post-emergence herbicides used to control weed seedlings in cereal, onion and garlic crops. This selectivity appears to come from the detoxification of the herbicides by a specific nitrilase. A gene coding for this enzyme has been isolated from Klebsiella ozaenae. The coding sequence of the gene was used to construct chimeric genes which were introduced into commercial tobaccos via Agrobacterium tumefaciens mediated transformation. Field trials show that transgenic tobacco plants are completely resistant to at least 16 times the bromoxynil dose used for effective weed control. This level of resistance was obtained in particular when the coding sequence is driven by the promoter region from a sunflower RuBisCO SSU gene. Taking these results into account, three kinds of field trials have been carried out since 1988 under the control of the French Commission de Génie Biomoléculaire : 1) Weed control program, bromoxynil or ioxynil alone or a mixture of both herbicides have been tested. A good weeding complementary efficacy of the mixture was observed on major weeds. 2) Metabolism studies show that the bromoxynil degradation pathway is similar in tobaccos and cereals. At harvest, the herbicide and metabolites from its degradation were below detection level. 3) Yield studies, including agronomic properties and raw material analysis, show that the quality of treated transgenic tobacco plants is similar to non treated non transgenic tobacco plants. Smoking of transgenic cigarettes did not show difference for the criteria studied. We propose that the oxynils are potential herbicides for transgenic tobacco crops because of : a - The high resistance of the tobacco plants to the oxynils; b - The unaltered behaviour in the field; c - Good post-emergence control of dicot-weeds. Registration experiments were planned for 1992/1993. We project commercial seed production of ITB 1000 OX during 1994.