A lot of powders with fine particle sizes and specific crystal habits are produced by precipitation processes, in which growth and morphology of the particles are controlled by a wide range of chemical agents. This study investigates alternative ways to limit crystal growth and influence crystal morphology. Experiments are carried out in an ultrasound levitator, where a single droplet can be suspended against gravity by an acoustic levitation force. The ultrasound leviator is a three-phase reactor, which allows the investigation of precipitation with applying specific shear forces at growing crystals within the droplet. The investigated system is calcium carbonate. By variation of physical reaction parameters as temperature and level of applied shear forces it is possible to obtain different crystal habits and morphologies. The morphology of calcium carbonate produced in the leviator can vary between prismatic, shell-like and spherical shape. Also the particle size distribution of the precipitated product is influenced. Increasing mechanical stress leads to a shift of the particle size distribution to smaller sizes. (c) 2005 Elsevier Ltd. All rights reserved.