The viscosity of partially crystallized Mg3Al2Si3O12 melts has been measured under uniaxial compression in the internal 1010--1013 poise as a function of the volume fraction of crystal. These inclusions are well-rounded spherulites of aluminous enstatite, having the same composition as the melt, and whose growth rate is negligible at the temperature of the measurements. The viscosity increases by less than 1 order of magnitude for crystal fractions &PHgr; of 40 vol % and remains Newtonian up to the maximum stress exerted, namely 1 kbar. The Einstein-Roscoe equation, &eegr;=&eegr;0 (1-&PHgr;/&PHgr;m)-n, provides very good fits to the measurements only if either the &PHgr;m or n parameter is allowed to depend on temperature. For modeling of magmatic processes, however, the widely recommended constant values &PHgr;m=0.6 and n=2.5 should be adequate. The rheology changes abruptly when the clustered spherulites begin to oppose shear deformation, at a crystal fraction of about 40 vol %. The viscosity becomes non-Newtonian, with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise. As long as the crystal fraction remains lower than 70 vol %, the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt. The deformation becomes again regular at low stresses with lower melt fractions, but samples undergo extensive fracturation along the direction of uniaxial stress. Similar rheology changes have been observed during the isothermal crystallization of Li2Si2O5 melts, which produces small ellipsoidal inclusions. These results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction, even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions. ¿ American Geophysical Union 1995