The flexure strength of partially-stabilized tetragonal ZrO2(Er2O3) single-crystal monofilaments manufactured by the laser-heated floating zone method was measured as a function of the environment (air versus water) and temperature (from 25° to 800°C) at loading rates spanning three orders of magnitude to ascertain their susceptibility to the environmental conditions. These mechanical tests were completed with parallel tests on fully annealed monofilaments (to relieve the thermal residual stresses induced during growth) and by detailed analysis of the fracture surfaces using scanning electron microscopy and micro-Raman spectroscopy. While environmental susceptibility of ZrO2(Y2O3) in previous investigations was always associated with the destabilization of the tetragonal phase, monoclinic phase was not detected on the fracture surfaces of the ZrO2(Er2O3) monofilaments and it was concluded that slow crack growth in this material at high temperature or immersed in water was due to stress corrosion cracking.