Micellar structure was determined by studying the extensibility of wood fibers in (1) water; (2) various organic solvents, especially alcohols; (3) aqueous solutions of liquids of different molecular size. Immersion in water or in the lower aliphatic alcohols increases the extensibility of the fiber, probably because the molecules of the liquid are able to enter the interstices between the micellae of the fiber, decreasing its resistance to extension. Other fluids have a similar, but less pronounced, effect, provided their molecules are not too large to enter the interstices; and if they are too large to have an effect on dry fibers, it may be possible for such substances to penetrate into the fiber when immersed in water. Penetration of involatile organic liquids is detected by determining the work required to stretch the fiber in the liquid and in an atmosphere in equi-librium with it; the 2 values are identical only if the reagent is unable to enter the fiber. Pore size is de-termined from the known size of the molecules of the liquid. Methods of determining size of micelles are given in detail. Results of the study show that the wool fibre is constructed from micelles lamellar in shape, arranged with their long axes parallel to the length of the fibre, and much (probably 10 times) longer than they are thick. The thickness of the micelles is of the order of 200 A. The inter-micellar distance in the case of the dry fibre is small and of the order of 6 A., but with wool fibres in water the distance is increased by swelling to 41 A. Thus the swollen fibre is far more easily accessible to reagents of high molecular wt. than the dry fibre. Knowing the thickness of the micelles and their approximate length, an approximate value for the internal surface of the fibre was calculated to be of the order of 1 X 106 cm.2/gm. Using the preceding values for the internal surface of the fibre and the inter-micellar distance, the total amt. of inter-micellar water adsorbed by wool from saturated air is 20.5% of the dry wt., a value in close agreement with that deduced from a study of the change in rigidity of wool with water adsorption[long dash]21.6%. Acids have a marked destructive action on the micelles of the wool fibre. They free the long-chain protein molecules within the micelles from one another to a certain extent, so that extension of the fibre is greatly facilitated. The change is reversible, and when the fibres are washed free from acid in water, their original elastic properties are recovered. Attack of the micelles by acid proceeds from the same thin edges which are attacked by Na2S solution.