The ultrasonic influence on the anti-solvent crystallization of roxithromycin is investigated. The effect of ultrasound on primary nucleation is studied by measuring the induction time and metastable zone width. The experimental results show that ultrasound has a significant effect on reducing the induction time. After applying ultrasound to the system, the change in the apparent nucleation order is small, however the nucleation rate constant (k(N)) increases significantly. The diffusion coefficient also changes significantly; however, the other parameters change slightly or remain constant within the ultrasonic field. This suggests that diffusion acceleration is the main reason for the reduction in the induction time. These observations also suggest that ultrasound can enhance the homogeneous nucleation of roxithromycin significantly. The influence of ultrasound on agglomeration and the crystal habit of roxithromycin is investigated in the second part of this contribution. Introducing ultrasound into the system reduces agglomeration and changes the crystal habit significantly. (C) 2004 Elsevier B.V. All rights reserved.