A process cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus includes first and second end covers, first and second bottom-surface portions to be guided by first and second main assembly guide portions, first, second, and third portions to be positioned by first, second, and third main assembly positioning portions, and a driving force receiving portion partly exposed through the first end cover between the first and third portions to be positioned and engaging with a main assembly driving force transmitting portion to rotate a process-cartridge developing member when the process cartridge is mounted to the apparatus.