The paper discusses the instability of the strain ratio and crystallographic orientation in face-centred cubic (fcc) real single crystals exposed to uniaxial straining. This behaviour can be described by the r ( ε ) function defining the variation of plastic anisotropy with strain, even in nominally stable crystals with ,  or  orientation. The instability is due to the error in the orientation of the sample axis ( α is the angle between the sample and the [ uvw ] nominal direction) and/or to microscatter of the crystallographic orientation in the whole volume of a single crystal (described by the half-width of the rocking curve ρ ). The instability and, consequently, texture in single crystals have been shown to depend on the stacking fault energy ( γ ). A quantitative evaluation of the instability of the crystallographic orientation in deformed single crystals has been proposed and its physical meaning revealed in experiments on low (Cu–Zn alloys, Ag)-, medium (Cu)- and high (Ni, Al)-stacking fault energy metals. Two different mechanisms should be assumed when investigating the influence of stacking fault energy on the instability of the crystallographic orientation in deformed fcc single crystals: changing from one mechanism to another, when γ varies from γ Ag to γ Cu .