The crimped structure of wool fibres is generally associated with a bilateral arrangement of ortho- and paracortical cells. The most obvious difference between these cell types is in the arrangement and relative proportions of microfibril and matrix proteins that constitute the fibre cortex. In the low-crimp fibres examined there is a poorer expression of bilateral cortical asymmetry compared with the high-crimp wools together with a higher proportion of intermediate-staining mesocortical cells. These mesocortical cells exhibit much more regular arrays of microfibrils than paracortical cells. It is suggested that the packing arrangement of microfibrils in all three cell types is basically hexagonal and the variation observed in mature cells is a function of the fibril:matrix ratio.