The invention concerns a process for producing middle distillates from a paraffinic feed produced by Fischer-Tropsch synthesis, using a hydrocracking/hydroisomerization catalyst which comprises: at least one hydrodehydrogenating element selected from the group formed by elements from group VIB and group VIII of the periodic table; 0.01% to 6% of phosphorus as a doping element; and a non-zeolitic support based on mesoporous alumina-silica.