A mechanism for cleaving optical fibers, comprising fiber cleaving means and gripping members arranged to grip an optical fiber and to apply a pulling force to put the fiber under tension whilst the fiber is cleaved, wherein the gripping member(s) C are arranged to eject a cleaved-off part of the fiber F into a receptacle 49 once the fiber has been cleaved, and/or the fiber F is bent during cleaving by means of a rotatable double anvil 47 widely separated from the gripping members.