This document specifies a gravimetric method and a vibroscope method for the determination of the linear density of textile fibres applicable respectively to: a) bundles of fibres; b) individual fibres. Useful data can be obtained on man-made fibres and, with less precision, on natural fibres. This document only applies to fibres which can be kept straight and, in the case of bundles, parallel, during test preparation. It is properly applicable when the fibres are readily freed of crimp. The methods in this document are not applicable to tapered fibres. The vibroscope method is not always applicable to hollow and flat (ribbon-like) fibres.