The deformation behaviour of single elementary flax fibres was investigated in an ESEM, using a modified loop test. Plastic deformation starts on the compressive side of the loop, whereas fibre failure occurs on the tensile side of the loop. The primary and the secondary cell wall show a different deformation behaviour. The primary cell wall breaks in a brittle manner, whereas in the secondary cell wall, due to its fibrillar nature, a coarse crack grows, bridged by fibrils. The secondary cell wall was found to split relatively easily along the length direction, indicating that the lateral strength of the fibre is lower than its tensile strength, which also accounts for the lower compressive strength of the fibre compared to its tensile strength.