Thin foils of niobium single crystals containing long screw dislocations with b =a/2 ⟨111⟩ have been irradiated at 900 keV in a high voltage electron microscope at room temperature. Climb of the dislocations under the electron beam lead to the formation of helical dislocations. In situ observations of their nucleation and growth are performed, and their nature is shown to be of interstitial type.