We present the results of three-dimensional numerical simulations thatinclude the effects of hydrodynamical forces and gas drag upon anevolving dusty gas disk. We briefly describe a new parallel, two phasenumerical code based upon the smoothed particle hydrodynamics (SPH)technique in which the gas and dust phases are represented by twodistinct types of particles. We use the code to follow the dynamicalevolution of a population of grains in a gaseous protoplanetary disk inorder to understand the distribution of grains of different sizes withinthe disk. Our ``grains'' range from metre to submillimetre in size.