SYNOPSISSome changes in the structure of cotton fibre which accompany mercerization are illustrated. The familiar swelling, rounding out, and partial untwisting of the collapsed fibre into a more nearly rod‐like shape have been investigated with both the light and the electron microscope. Shrinkage and compacting of the primary‐wall network to provide a constricting casing about the fibre, crimping of the microfibrils of the secondary‐wall cellulose, and ultimate smoothing out of the wrinkled primary wall have been observed. A technique for expansion of the fibre wall into layers further demonstrates the altered structure of the cotton fibre after tension‐mercer