Adopting activity theory as a theoretical and methodological framework, this case study illustrates how a teaching and learning situation is planned and implemented over a series of nine 75-min biology classes by a high school science teacher in the context of pedagogical reform. The object of this study emerges within a favourable context of science education curricular reform in Quebec, Canada. By examining the interaction between the poles of an activity system sharing the same object, this case study illustrates how one teacher's teaching practice is redefined and how some aspects of her teaching personality orient the ways in which she contextually mobilizes new tools and members of her school community in order to implement an awareness campaign on the risks of tanning salons.