In this paper, we analyse experimental results concerning the transverse mechanical properties of PA6.6 monofilaments. Three diameters of such monofilaments (120, 230 and 400 mu m) were compressed at seven different speeds, varying from 1 to 5,000 mu m/min, until a maximum force of 2,000 N. The results show a high degree of anisotropy: the transverse Young's modulus is less than one-tenth the value of the longitudinal one. In general, as the speed increases, more energy is needed to compress the monofilament and the curves representing the evolution of the contact width as a function of the force per unit length are in descending order. This happens in the plastic region, as long as the phenomenon is well stabilised, and indicates a viscoelastoplastic behaviour of PA6.6 monotilaments in the transverse direction. Transverse relaxation tests confirmed this behaviour. However, the curves representing the evolution of the diameter as a function of the force per unit length are not always in descending order as the speed increases. These curves also show a change in the optical properties of the compressed monofilament. Scanning Electron Microscope photos of transversely compressed PA6.6 monofilaments revealed that the monotilament seems to fibrillate inside its core.