Unprocessed and mercerized cotton fibers were treated with commercial crude cellulase. The changes in the dyeability and structural features of the fiber due to cellulase treatment were studied. The dyeability was examined in terms of uptake of three reactive dyes and the apparent affinity of Congo Red to cotton fiber. The dyeability of the unprocessed fiber was assumed to be influenced by some impurities present in it. This fiber probably resembled polynosic fiber in molecular aggregate at a certain stage of hydrolysis. Mercerized cotton showed a similar pattern in dyeability as weight loss increased, regardless of dye species. Enzyme more easily penetrated the mercerized fiber than the unprocessed fiber. Cellulase treatment influenced the X-ray crystalline reflection pattern for the mercerized fiber but nominally influenced that for unprocessed fiber. Scanning electron micrographs revealed that cellulase treatment caused swelling of the fibrils. They also revealed that the disordered regions between the fibrils in the secondary walls were removed at low weight loss for the unprocessed fiber. The mercerized fiber at high weight loss had large cracks oblique to the fiber axis and showed no individual fibrils in the secondary wall. The primary wall was removed in the initial stage of hydrolysis for both the unprocessed and mercerized fibers.