The defect structure of naturally deformed augite containing enstatite lamellae has been investigated by conventional and high resolution transmission electron microscopy in order to obtain information on the deformation mechanisms of these important rock-forming minerals. The aggregate under investigation originates from a coarse-grained websterite dyke of the Balmuccia Massif which is located within the Ivrea Zone (NW-Italy). Dyke formation may have taken place during intrusion of the peridotite massif into the lower crust at temperatures of about 1000°C. Deformation of the pyroxenite dykes may be due to subsequent tilting of the Ivrea Zone at lower temperatures (about 650°C). Cooling of the augites led to exsolution of orthopyroxene, clinoamphibole and two generations of pigeonite lamellae. The microstructure observed in the augite matrix and the enstatite exsolution lamellae consists of dislocations, planar faults and subgrain boundaries. Burgers vectors and dislocation line directions in augite indicate the activation of the (100)[001], {110} 1 2 <110>, {110} 1 2 1/2<112>, (100)[010], (010)[100], (010)<101> and {110}<111> slip systems, the first two being the most active. Dislocation reactions are common, as well as heterogeneous precipitation of pigeonite at [001] dislocations. The subgrain boundaries observed are { hkO} tilt boundaries with [001] tilt axis and (010) twist boundaries. Occasionally (100) twin lamellae exist. The dislocations in the deformed enstatite lamellae have [001], [010] and [100]Burgers vectors, as well as combinations of these. In general, all dislocations are dissociated. The subgrain boundaries observed are { hkO} tilt boundaries with [001] tilt axis. The planar faults produced by dissociation of [001], [010] and <111> dislocations in augite are stacking faults (SFs) on (100), (010) and {110}, respectively, that of 1 2 <112> and <101> dislocations are chain multiplicity faults (CMFs) on (010). The planar faults produced in enstatite are either SFs on (100) or CMFs on (010). The SFs represent layers of proto- or clinoenstatite. The CMFs in both phases are amphibole-type defects, the formation of which requires a certain solubility of OH ions in the lattice. Broadening of the CMFs occurs by movement of partial dislocations along the fault leading to clinoand orthoamphibole lamellae. Diffusion of the necessary ions most probably is along the dislocation core of the partials. The main deformation mechanisms in augite and enstatite are dislocation creep and the stress-induced ortho/clinoenstatite inversion, respectively. Dislocation dissociation hinders dynamical recovery by climb and cross slip, exsolution leads to precipitation hardening. These processes may account for the different rheological behaviour between pyroxenes and olivine. A comparison of the microstructure in orthopyroxene lamellae deformed within the augite matrix with that in orthopyroxene grains deformed within an olivine environment shows that additional independent slip systems are activated if orthopyroxene is embedded in a less ductile matrix. Nevertheless, similar to clinopyroxenes, five independent slip systems required for full strain compatibility are not activated because slip on planes cutting the chains does not take place.