Abstract: A multiple-stage catalytic conversion system in which a hydrocarbon charge stock is countercurrently reacted in a plurality of catalytic reaction zones, in all of which the catalyst particles are downwardly movable via gravity-flow. The charge stock, in the absence of added, or recycled hydrogen, is reacted serially in the reaction zones in the order of increasing catalyst loading, the product ultimately being recovered from the effluent emanating from that reaction zone (1) into which fresh, or regenerated catalyst particles are introduced and, (2) which contains the greatest quantity of catalyst particles. Catalyst particles are transferred from one reaction zone to another in the order of decreasing catalyst loading, ultimately being withdrawn from the system through the reaction zone containing the least amount of catalyst particles.