The surface morphology of dewaxed jute and of dewaxed and scoured flax and ramie fibres, and the effect of chemically modified morphologies were compared using scanning electron microscopy (SEM). Limited oxidation results in the removal of surface impurities of the bast fibres, producing strands which are clearly distinguishable in the SEM. Treatment of the oxy-fibres with excess phenol (P) and formaldehyde (F) at pH 8 leads to permanent in situ deposition of P-F resin moieties, which makes the strands less clearly visible. Modification of the P-F treated oxy-fibres by vinyl grafting leads to further masking of the fibre strands due to measurable vinyl deposition; in the SEM the fibre strands appear closely cemented together by the grafted-on vinyl polymer. On exposure to a standard microbiological degradative environment, damage to the fibre strands takes place in the order jute ≫ flat ≥ ramie; oxy-jute ≫ oxy-flax ≥ oxy-ramie. Each fibre system suffers little microbiological degradation, thereby showing high rot resistance, when the respective oxy-fibres are modified by P-F treatment and also by vinyl grafting in a subsequent step. At this stage the difference between the three fibre systems in rot resistance becomes slight. The SEM observations are supported by analysis of tensile strength (TS) and retention of TS after exposure to a standard microbiological degradative environment. © 1990 Chapman and Hall Ltd.