A cartridge detachably mountable to a main assembly of an electrophotographic image forming apparatus, includes a developing member for developing an electrostatic latent image formed on an electrophotografic photosensitive member, a developer accommodating portion for accommododating a developer to be used for developing an electrostatic latent image by the developing member, a cartridge locking portion for locking the cartridge with a main assembly locking portion provided in the main assembly of the apparatus to prevent the cartridge from disengaging from the main assembly of the apparatus when the cartridge is mounted to the main assembly of the apparatus, and a relasing member for releasing the cartridge locking portion to release the cartridge from the main assembly locking portion when the cartridge is to be removed from the main assembly of the apparatus.