A single crystal of the j 0 -AlPdMn phase was plastically deformed in a three-point bending geometry. The deformed crystal shows a periodic lattice of phason lines. In a high-resolution electron microscopy (HREM) study a novel dislocation type, termed metadislocation, was observed for the first time. The magnitude of its Burgers vector is 171 Å. At the core of the metadislocation a dislocation of the j 0 structure is found. For the first time its structure and Burgers vector were determined by HREM. In magnitude and direction it is equivalent to the most common Burgers vector in the icosahedral quasicrystal. The structural relationship between the dislocation and the metadislocation is discussed.[S0031-9007(99)08984-X] PACS numbers: 61.72. Ff, 61.16.Bg, 61.44.Br, 62.20.Fe The predominant plastic deformation mechanism in crystalline solids is governed by the motion of dislocations. Despite the lack of translational symmetry in quasicrystals dislocations exist in them [1], and as in crystals moving dislocations carry plastic flow. This has been shown in high temperature deformation experiments for quasicrystals of icosahedral [2] and decagonal [3] structure. The structure of a quasicrystal, quasiperiodic in the physical three-dimensional space, can be described as a periodic structure in a higher dimensional hyperspace, which is a six-dimensional (6D) space in the case of an icosahedral quasicrystal. The Burgers vectors of dislocations in such a phase can then be described as 6D vectors. In contrast to periodic crystals, however, icosahedral quasicrystals show no work hardening. They even become softer the more they are plastically deformed [4]; i.e., the flow stress decreases with increasing strain.In the compositional vicinity of quasicrystalline phases one often finds periodic phases with large unit cells, having cell parameters that can be expressed as a function of the hyperspace cell parameter associated with the corresponding quasicrystal and the golden mean t ͑1 1 p 5 ͒͞2. These phases, termed approximants, are periodic in structure and therefore crystals, and they are close in chemical composition, local order, and physical properties (electrical and thermal conductivity, magnetic behavior, etc.) to quasicrystals [5]. One should therefore also expect to find a similar behavior of approximants regarding the plastic deformation mechanism. On the other hand, due to the large cell parameters in approximants, a complete dislocation in such a phase would necessitate a Burgers vector of more than 10 Å in magnitude. Such a large Burgers vector would involve very high defect energies compared to what is observed in simple crystals, making its existence improbable.As yet, no experimental investigations of the plastic deformation behavior of approximants have been published. Moreover, no observations of dislocations in approximants have been reported. Klein et al. [6] proposed a geometrical model for the plastic deformation of an approximant (j 0 phase [7]) of the icosahedral Al-Pd-Mn phase which does not involv...