This article presents the results of data-based research carried out in the field of teacher training of French L2. The research dates back to 1994 and originates from spoken French data gathered from French and Flemish speakers, known as the Corpus LANCOM. LANCOM was used to train Flemish teachers of French in the late nineties and to define the attainment targets of the Flemish curriculum for French. In 2008 Delahaie carried out research on how three Flemish teachers of French taught a communicative spoken French to their classes. She shows that only the teacher who benefited from the use of Lancom in her training succeeds in making her pupils acquire authentic spoken French, even though by lack of practice she is a less fluent speaker of French than her two colleagues. This data experiment addresses the issue of how corpora can be used to meet teachers' needs when they have to deal with the thorny issue of spoken French ability.