Rheologies of the lower continental crust and overall contrast with the underlying upper mantle likely control the evolution of first-order structures. Stress and strain partitioning between natural shear zones and wall rock(s), foliation development, flow mechanism(s) and constitutive relations of rocks that make up the lower crust are poorly known. We have undertaken an experimental study in which an orthopyroxene granulite is deformed in simple shear. The starting material is a three-phase (plagioclase-orthopyroxene-amphibole) equigranular granulite, previously used in coaxial experiments. Wafers cut at 45° to the long axis of cores were placed between Zircoa forcing blocks that have been cut obliquely, at 45°, at one end. All experiments were run at 1.2 GPa (Pc) and temperatures of 650°–950°C. Specimens have been strained up to a maximum of γ = 2.6, at shear strain rates of 7 × 10−5 to 7 × 10−7 s−1. Most shear stress-shear strain curves have a similar form. They are characterised by an initial linear response that is followed by a work-hardening region that leads to a level of quasi-steady state. The quasi-steady-state behaviour is followed by a gradual decrease in flow stress to a second quasi-steady-state stress level, that very gradually decreases with increasing strain. Optical and TEM examination of deformed samples indicates that an amphibole foliation develops during the first quasi-steady-state stress level by fragmentation and alignment of amphibole; plagioclase and orthopyroxene undergo minimal crystal plastic strain. At the onset of the second quasi-steady-state stress level, plagioclase develops neoblasts, orthopyroxene develops clinopyroxene lamellae and the amphibole foliation is better developed. With increasing strain, compositional lamination develops, as in natural mylonites, with extensive neoblast development in plagioclase and orthopyroxene, and cataclastic grain refinement in amphibole. Strength data, measured at the onset of the second quasi-steady-state level, fit a power law with Q = 203 kJ/mole and n = 2.3. These Q and n values are significantly lower than those determined under similar conditions in previous coaxial experiments. With increasing shear strain, as under natural conditions, there is a significant grain-size reduction, that may lead to a switch in flow mechanism from grain-size insensitive flow, at low strain, to grain-size sensitive flow at higher natural strains.