In situ transmission electron microscopy (TEM) observations were performed on yttria-stabilized zirconia during caesium (Cs) ion implantation at room temperature. Apparition of defect clusters is observed. The concentration of the latter increased with the Cs ion fluence. Until the higher fluence (2 x 10(16) cm(-2)), nothing else was observed except the overlapping of these defect clusters. At the higher fluence, Cs ion implanted thin sample was annealed between 600 and 1200 K. Only the recrystallization of cubic zirconia occurs during annealing; no other compounds were formed. The TEM results are compared to previous results obtained from Rutherford backscattering and channelling ion beam analysis techniques. (C) 2008 Elsevier B.V. All rights reserved.