Plant fibres have been used since ancient times in numerous applications ranging from nets and basketry to textiles for clothing. A proper identification of plant fibre remains in archaeological excavations provides important information about resource exploitation and agriculture. In order to identify plant fibres, a series of fibre features—cross-section diameter, lumen diameter, dislocations (nodes), cross markings, cross-section shape and lumen shape—have been defined and characteristic traits of these features for different fibre types established. How suitable these traits really are for fibre identification has been a matter of debate. To resolve this issue, we have performed a systematic investigation of typical textile bast fibres: flax, nettle and hemp. We have investigated cross-section diameter, lumen diameter, dislocations (nodes) and cross markings using standard compound, white light transmission microscopy. Our investigations show that all the traits that are considered characteristic for one type of plant fibre can also, on occasion, be found in other types. This demonstrates that an investigation of the traits listed above is not sufficient to ensure a correct identification of the plant fibre material; in particular, when only a small amount of material is available. This is often the case in archaeological excavations.