An apparatus is described for measuring the force to cut single fibres. Cutting wires are pressed under a gradually increasing load at right-angles to a fibre mounted under constant tension on a plane anvil and the cutting load is determined. The quantitative relations between cutting load and fibre diameter, fibre tension and diameter of the cutting wire are given for wool, nylon, and Terylene. For these fibres, cutting load is a broad reflection of tensile strength.