In this,paper, a model was constructed in order to simulate the behaviour of slow growing crystals in high viscosity systems in a tower crystallizer. This type of crystallizer, ideally,. is an application of the plug-flow concept. Plug-flow conditions are an advantage when a narrow crystal size distribution is the goal desired. The model was based on the following assumptions: steady state operation,, no nucleation or growth rate dispersion, no breakage, size-dependent growth and settling of crystals. A simulation example for sucrose is presented although the model applies to other similar systems.