In the hydrodesulfurization of residual oil the amount of hydrogen consumed per atom of sulfur removed is relatively low until the desulfurization becomes deep, whereupon the amount of hydrogen consumed per atom of sulfur removed becomes relatively high. The present invention provides a multistage hydrodesulfurization process capable of producing products of low sulfur level while avoiding deep desulfurization of the heavy portion of the residual oil so that hydrogen consumption is diminished. The feed oil is fractionated to provide distillate and residual fractions. The residual fraction and hydrogen are charged to an upstream catalytic hydrodesulfurization stage. A portion of the upstream stage effluent stream is diverted from the process for use as refinery fuel and the remaining portion of the upstream stage effluent stream is charged to a downstream catalytic stage together with the feed distillate oil and hydrogen. The interstage diminution of the residual oil fraction provides a non-aliquot distillate-residual oil second stage feed stream which is relatively enriched in distillate oil. Since the sulfur in distillate oil is considerably less refractory than the sulfur in residual oil, the resulting dilution of the residual content of the downstream stage feed stream reduces hydrogen consumption by diminishing the depth of residual oil hydrodesulfurization required in the downstream stage to produce a low sulfur product.