The study describes a teaching action undertaken in the belief that the use of methodologies based on active and cooperative learning could obviate some of the most worrying deficiencies in current scientific teaching, while at the same time supporting the validity of the constructivistic theory that prompted them. A teaching action on genetically modified organisms (GMOs) was planned which involved the setting up of tools (diagnostic tools, strategies imbued with problem-based learning and concept cartoons), a teaching sequence and laboratory materials (replica-plating aimed at the recognition of transformed bacteria, Nicotiana cultures, electrophoresis comparing protein patterns of GM and non-GM plants). It was then carried out in 10 classes of six upper secondary schools (three specialising in classical and three in scientific studies) in Rome, with a total of 144 students ranging in age from 16 to 19. The written texts of students' views and arguments were analysed statistically and conceptually, which led to an overall positive evaluation of the teaching activity, at least as regards the acquisition of scientific terminology and argumentation. The action can also be considered effective with respect to scientific education as well as overall intellectual and social maturity. Though the structure and the organisation of the schools involved were very different, autonomous inquiry, group work, and plenary discussion were not impeded in any way. (Contains 2 tables, 4 figures, and 3 footnotes.)