The rheological behaviour of synthetic crystal-bearing magmas containing up to 75 vol.% of crystals (0≤ ΦS ≤ 0.75) has been investigated experimentally at a confining pressure of 300 MPa and temperatures between 475 and 600°C at shear rates between 10-4 and 2x10-3 s-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 75 vol.% of Al2O3 sieved (45 < Ø < 90 µm) crystals. Shear viscosity measurements were performed in torsion in a Paterson gas-medium apparatus. For each composition, the rheological data were fitted to a power law of the form: (1) where is shear rate, is shear stress , T is temperature, R is the gas constant, and A, n and Q are empirical parameters defined as the preexponential term, the stress exponent and the activation energy, respectively. The rheology is found to be newtonian (n=1) for pure hydrated melt and 16 vol.% of crystals. At higher crystal contents, the magmas behave as pseudoplastic materials, and exhibit shear thinning behaviour with stress exponents of 1.3, 1.9 and 2.8 for 34, 54 and 65 vol.% of crystals, respectively. The activation energy seems to be nearly independent of crystal fraction and has a mean value of 231 kJ.mol-1. The Einstein-Roscoe equation adequately estimates viscosities of the crystal-bearing magmas at low crystal contents (ΦS ~≤ 0.25), but progressively deviates from the measured viscosities with increasing crystal content as the rheological behaviour becomes non-newtonian. A modified power-law formulation incorporating crystal content as a variable allows a fit to the experimental data set over the whole range of conditions.