In order to design human-machine dialogue systems using speech input as well as written intput, it is necessary to study the influence of the machine not only on the user's lexicon and syntax, but also on his dialogue behaviour. Dialogue is not handled between a man and a machine in the same way as an interpersonal dialogue. This paper is based on the linguistic study of a corpus obtained by simulating a machine in an actual human-machine dialogue situation: it deals with a task-oriented dialogue recorded by a telephone information service of the French National Railway Company (SNCF), for train timetables (schedules). In the paper, we focus on the basic elements of dialogue behaviour induced by the machine, namely the respect of "turns" (the way one participant gives or takes control of the communication channel to or from the other): in the experiment carried out at the SNCF, naive users spontaneously accepted this constraint. We believe that such behaviour reflects a. hierarchically-structured dialogue which we formalize with a set of defined and reduced symbols.