A fluidized bed crystallization process was investigated experimentally, characterized by a continuous withdrawal of a suspension containing target particles having narrow size distribution. Three industrially relevant substances were applied in the study, namely L-glutamic acid, L-asparagine monohydrate and ortho-aminobenzoic acid. A continuous supply of seeds, required for the establishment of a steady state, was achieved by applying ultrasonic comminution of bigger crystals and agglomerates withdrawn from the bottom of the conical shaped lower section of the fluidized bed crystallizer. The influences of the flow rate and the initial seed crystal sizes on the generated product size distribution were studied. Steady state operation was achieved, i.e. the median sizes of the collected product crystals remained constant. A simple empirical force balance model based on the Richardson-Zaki relation could be successfully applied to explain main features of the observed internal crystal size distributions. (C) 2014 Elsevier Ltd. All rights reserved.