The design of a parameter-dependent controller to reject periodic mould level fluctuations at a thin slab caster is presented. These fluctuations can be related to bulging of the solidified steel shell between the supporting rolls. They deteriorate the steel quality and prohibit high speed casting. A state of the art controller has been designed to suppress these casting speed dependent fluctuations and is based on a H∞-optimal controller design technique. The controller has been implemented at caster of the Corus Direct Sheet Plant in IJmuiden. Measurement results show a significant reduction of these periodic mould level fluctuations.