In many cases of precipitation the product comprises of two or more polymorphs, either due to their simultaneous nucleation or due to transformation of a metastable polymorph. These polymorphs may have different growth rates as well as aggregation tendencies. In the present work the population balance equation is applied to predict the size distribution of the precipitate particles allowing for these features. The Hounslow method for discretization of the population balance equation is modified for improved stability and accuracy, and used to solve the model for polymorph precipitation in a continuous system. The effect of the aggregation tendencies of the polymorphs, as well as of the ratios of their nucleation rates and of their grow rates, on the particle size distribution and product composition is studied.