A cylinder lock having a removable barrel that includes a control tumbler. The barrel is retained in the housing of the lock by a nib that is formed on either the barrel or the interior of the housing. A longitudinal nib slot is formed on the other of either the barrel or the interior of the housing. The nib and the nib slot are aligned with one another only when the barrel is in an unlocking position for permitting the barrel to be removed from the housing. In this manner, the barrel may be removed from the housing only when both the barrel is in the unlocked position and when a special control key has been inserted into the barrel for withdrawing the control tumbler into the barrel.