This work deals with the fibrillation of the cortex of Merino wool by successive freezing (freezing followed by thawing) in various swelling liquids. The freeze-thawing experiments were carried out on native and decuticled wool fibers using water, n-propanol/water, dichloroacetic acid, and formic acid as swelling liquid at freezing temperatures of -78°C and -196°C. The results on native wool showed that the fibers were finely cut in round slices under suitable conditions, but no fibrillation of the cortex occurred under the experimental conditions. For decuticled wool, on the other hand, fibrillation of the cortex was caused by repeating the freezing cycles of 200 in dichloroacetic acid or in formic acid. The orthocortical and paracortical cells were separated from the disintegrated cortex by using a difference in density between both cortical cells, and their amino acid compositions were compared with those reported in literature.