Decagonal single quasicrystals of the composition Al₇₀Ni₁₅Co₁₅ have been deformed in situ in a high-voltage transmission electron microscope at 730 ^°C along the 10-fold periodic axis to directly observe the dislocation motion. The deformation is carried by stress-assisted climb of dislocations with periodic Burgers vectors. These dislocations may also glide and move by a combination of glide and climb. Dislocations with Burgers vectors with components in the periodic and quasiperiodic directions probably move under the action of a chemical force. The observations are interpreted by a model established by P. Schall et al. under consideration of the activation parameters of macroscopic deformation and by analogies with the behavior of icosahedral quasicrystals.