The rheological behaviour of a low-rank coal-water slurry from Lochiel, South Australia, has been studied as a function of solids concentration, particle size and size distribution. Coal slurries consisting of particles finer than 45 mu m suspended in water were found to exhibit a wide spectrum of flow behaviour ranging from Newtonian at low solids concentrations to shear-thinning and viscoplastic with a yield stress at higher concentrations. By adding a narrow-sized coarse coal fraction (208-279 mu m) to the fine coal slurry, the flow characteristics of the slurry, at a fixed total solids concentration, could be changed significantly with a substantial reduction in the slurry viscosity. An optimum coarse-to-fine particle ratio of 40:60 exists at which the slurry is Newtonian with a minimum viscosity of about 5 times lower than the viscosity of a slurry containing only fine particles at the same solids concentration. A further reduction in the slurry viscosity of 50% was achieved by adding a second coarse coal particle (279-325 mu m) to the bimodal slurry. The significant improvement in the rheological behaviour with changing the particle size distribution may be explained in terms of spatial rearrangement of the particles, and an apparent dilution effect. The results obtained in this study indicate that, with a careful control of the particle size distribution, it is possible to prepare an optimum coal-water slurry which has a low viscosity but with high solids loadings.