Plastic deformation experiments have been performed on Czochralski-grown decagonal Al-Ni-Co single-quasicrystals at temperatures between 780 and 860°C. Compression tests at a strain rate of 10⁻⁵s⁻¹ with different orientations of the compression axis relative to the tenfold quasicrystal direction show an anisotropy of the plastic behaviour. If the compression axis is oriented parallel to the tenfold direction multiple slip and weak work hardening is observed. If the compression axis is tilted by 45°, single-slip conditions and deformation softening are found. Microstructural investigation by transmission electron microscopy indicates that plastic deformation is mediated by a dislocation mechanism. The results are interpreted in terms of a model in which the chemically ordered columnar clusters of the decagonal structure represent rate-controlling obstacles.