The influence of weathering on the chemical and physical properties of wool fibers has been investigated. The weathered fiber tips are modified to a great extent by weathering: this is shown in changes in solubilities and in the amino acid composition. As a result of photochemical degradation, fibers show a decrease in tenacity and extension as well as in compressional resistance. Weathered fiber tops showed, in spite of removal of a great number of scales, a better feltability than the fiber roots. On the basis of these results a theory of felting is presented and discussed.