The rheological behaviour of synthetic crystal-bearing magmas containing up to 76 vol.% of crystals (0≤ϕ_S≤0.76) has been investigated experimentally at a confining pressure of 300 MPa and temperatures between 475 and 1000 °C at shear rates between 10⁻⁴ and 2×10⁻³s⁻¹. Starting hydrated crystal-bearing glasses were synthesized from a dry haplogranitic glass (Qz₃₆Ab₃₉Or₂₉) and 2.5 wt.% water mixed with 0 (pure hydrous melt), 16, 34, 54, 65 or 76 vol.% of Al₂O₃ sieved (45<Ø<90 µm) crystals. Shear viscosity measurements were performed in torsion (simple shear) in a Paterson gas-medium apparatus. For pure hydrated melt and for 16 vol.% of crystals, the rheology is found to be Newtonian. At higher crystal contents, the magmas exhibit shear thinning behaviour (pseudoplastic). The Einstein–Roscoe equation adequately estimates viscosities of the crystal-bearing magmas at low crystal contents (ϕ_S≤~0.25), but progressively deviates from the measured viscosities with increasing crystal content as the rheological behaviour becomes non-Newtonian. On the basis of a power–law formulation, we propose the following expression to calculate the viscosity as a function of temperature, crystal content and applied stress (or shear rate):where γ. is shear rate (s⁻¹), τ is shear stress (MPa), Φ is the crystal volume fraction, T is temperature (K), Φ_m is the relative maximum packing density, R is the gas constant, Q=231 kJ mol⁻¹ is the activation energy of the viscous flow and A₀, K, K₁ and K₂ are empirical parameters.