Under the action of the illuminating electron beam in an electron microscope, dislocations in Al-Mg alloys start to move during examination. It was found that screw dislocations move and change easily their moving direction producing many double cross-slip traces, and that slip and cross-slip first occur on those {111} planes lying close to the position at 45°with the foil surface. The cross-slip direction or the Burgers vector of the screw dislocation is the α/2 making an angle of 60-80° with the foilnormal. These observations are in good agreement with the fact that the stress is assumed to be tangential to the foil surface during bending caused by heating.The moving dislocation may through its stress field induce the movement of other dislocations lying close to its path, and on the other hand, may itself be stopped or forced to change its moving direction by the repulsive force of other dislocations.