It is well known that ultrasonic field is generally used in the cleaning of metal surfaces. Recently, there have been some attempts for the use of ultrasonic treatment in the mineral industry, especially in flotation and separation of fine particles from gases or liquids. Ultrasonic energy is used as a first time in the sedimentation of clay in this study. Laboratory scale sedimentation experiments are performed in natural and ultrasonic media in order to determine the sedimentation characteristics of clay samples in different settling environments. The effects of several parameters such as pulp density, flocculant addition and ultrasonic treatment on the sedimentation of clay are investigated and the optimum settling conditions are determined. It is found that ultrasonic treatment, applied during the sedimentation, positively affected the sedimentation of clay and increased the settling rates by lowering the settling time. (C) 2003 Elsevier Science Ltd. All rights reserved.