Given a closed orientable Lagrangian surface L in a closed symplectic four-manifold (X,ω) together with a relative homology class d ∈ H2(X,L;Z) with vanishing boundary in H1(L;Z), we prove that the algebraic number of J-holomorphic discs with boundary on L, homologous to d and passing through the adequate number of points neither depends on the choice of the points nor on the generic choice of ...