This paper presents results from a study of fibres extracted from Zostera marina eel-grass collected from the Baltic coast. This species of sea-grass is shown to contain small diameter (around 5 mu m) fibres composed of similar to 57% cellulose, similar to 38% of non-cellulosic polysaccharides (mainly xylan) and similar to 5% of residual matter so-called Klason lignin. This composition is quite different to that of commonly used terrestrial fibres. Single fibre stiffness values up to 28 GPa were measured. This stiffness combined with a low density could provide an attractive reinforcement for composite materials, and may be particularly suitable for use in bio-degradable structures.