Résumé : The linear perturbation analysis of morphological and thermosolutal instabilities at the growth front during directional solidification of a binary alloy is generalized to the case where bulk convection exists in the basic state, as is encountered in most cases, in practice. the model assumes a finite and deformable solute boundary layer DELTA . The compatibility condition is obtained in analytical form for the marginal case of exchange of stabilities. The present model is shown to be consistent with literature results in the limiting case of infinite DELTA .