A cartridge loading device 10 for magnetic tape cartridges includes a first member 12, a second member 14 movable relative to the first member from a first position to a second position and a guide arrangement for guiding movement of the second member between its first and second positions. The guide arrangement includes a projection 38 received in an elongate guide track 32 defining a line of movement having a first end that defines the first position of the second member 14 and a second end that defines its second position. The cartridge loading device also has a locking system that includes a pivotable locking member 82 engageable with the projection 38 for locking the second member in its first position. The cartridge loading device can receive two different sizes of cartridge that correspond to the two positions of the second member. The locking member is actuable by insertion of a cartridge having the larger of two sizes into the device to release the projection to permit movement of the second member from the first position to the second position. The pivotable locking member 82 has a pivot point 92 disposed on a line that is in line with at least a first portion of the line of movement. The first portion extends from the first end of the line of movement.