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. The procedures can be applied only to fibres which can be kept straight and, in the case of bundles, parallel, during test preparation. These methods are properly applicable when the fibres are readily freed of crimp. They are not applicable to tapered fibres. NOTE The vibroscope method may not be applicable to hollow and flat (ribbon-like) fibres.