Note sur le lieu discriminant d'une hypersurface de bi-degré $(m,n)\mathrm{.}$

We present a new topological method to study the discriminantal loci of an algebraic variety defined in a product of projective spaces. Our approach relies on an efficient use of groupoid to describe the monodromy. As an example, we treat here the discriminantal loci of an hypersurface of bi-degree $(m,n)\mathrm{.}$