Cubic ZrO₂ single crystals fully stabilized with 9.4 mol% Y₂O₃ were strained in situ in a high-temperature tensile straining stage inside a high-voltage electron microscope. Straining was usually performed at 1150^°C. One experiment started at 1150^°C was interrupted before it was continued at 870^°C. For investigating the dependence of the deformation process on the activated slip systems, specimens with <112> and <100> tensile axes were used. Specimens with a <112> tensile axis, where single slip on one cube slip system is preferentially activated, were prepared to have 111) or 110) foil surfaces. Of the specimens with a <100> tensile axis, where slip on cube planes is suppressed, 001 foil surfaces were chosen. The observed dislocation structures and especially the dynamic behaviour of dislocations recorded on video taps furnished information on the mode of dislocation multiplication and on the mechanisms controlling the flow stress. The analyses of the dislocation density provided a value of the long-range stress component. Estimating the local shear stress from the bowed-our dislocation segments yielded a semi-quantitative explanation of the macroscopic flow stress for the deformation on the cube slip plane at 1150^°C. In this temperature range the athermal dislocation motion is in accordance with the very low strain rate sensitivity measured in macroscopic tests. At 870^°C, dislocations are pinned at localized obstacles. Dislocations on non-cube systems experience a lattice friction, even at 1150^°C.