Cotton fibres possess several β-glucanase activities which appear to be associated with the cell wall, but which can be partially solubilised in buffers. The main activity detected was that of an exo-(1→3)-β-D-glucanase (EC 3.2.1.58) but which also had the characteristics of a β-glucosidase (EC 3.2.1.21). Endo-(1→3)-β-D-glucanase activity (EC 3.2.1.39) and much lower levels of (1→4)-β-D-glucanase activity were also detected. The exo-(1→3)-β-glucanase showed a maximum late on (40 days post-anthesis) in the development of the fibres, whereas the endo-(1→3)-β-glucanase activity remained constant throughout fibre development. The β-glucanase complex associated with the cotton-fibre cell wall also functions as a transglucosylase introducing, inter alia, (1→6)-β-glucosyl linkages into the disaccharide cellobiose to give the trisaccharide 4-O-β-gentiobiosylglucose.