Thin sections of the rachis of regenerating follicles of pigmented fowl feathers and of mature non-pigmented seagull feather rachis, embedded in methacrylate and Araldite respectively, were studied in the electron microscope. The late stages of development of keratin fibrils were examined in OsO(4)-fixed follicle material, and after poststaining with lead hydroxide the keratin aggregates were found to be composed of fine microfibrils approximately 30 A in diameter apparently embedded in a matrix material which had absorbed the lead stain. The centre-to-centre separation of the microfibrils was of the order of 35 A. After bulk treatment by reduction with thioglycollic acid, OsO(4) staining, and poststaining with lead hydroxide, a similar microfibrillar fine structure was observed in mature rachis. Only after lead staining could the microfibrils be delineated, and their diameter and separation were similar to that found in the keratin of the follicle. It is suggested that feather keratin resembles alpha-keratins in consisting of microfibrils embedded in an amorphous protein matrix. However, in comparison with alpha-keratins, the microfibrils are much smaller in diameter, their arrangement is less orderly, and on the basis of the reactions towards the electron staining procedures, the cystine content of the matrix appears to be not greatly different from that of the microfibrils. The significance of a microfibrillar constitution of feather keratin is discussed in relation to current structural models for this fibrous protein deduced from x-ray diffraction studies. The boundaries between the component cells of feather rachis are desmosomal in character and similar to those of related keratinous structures and a number of different types of cells; the melanin granules are dissimilar to those of mammalian epidermis in their apparent lack of melanin-protein lamellae.