In this paper, we study the notion of marginal independence between two sets of variables when uncertainty is expressed by belief functions as understood in the context of the transferable belief model. We define the concepts of non-interactivity and irrelevance, that are not equivalent. Doxastic independence for belief functions is defined as irrelevance and irrelevance preservation under Dempster's rule of combination. We prove that doxastic independence and non-interactivity are equivalent. Keywords. Belief functions, Transferable Belief Model, Non-interactivity, Irrelevance, Doxastic Independence.