The design of a parameter-dependent controller to rejectperiodic mould level fluctuations at a thin slab caster ispresented. These fluctuations can be related to bulging ofthe solidified steel shell between the supporting rolls. Theydeteriorate the steel quality and prohibit high speed casting.A state of the art controller has been designed to suppressthese casting speed dependent fluctuations and is basedon a H∞-optimal controller design technique. The controllerhas been implemented at caster of the Corus Direct SheetPlant in IJmuiden. Measurement results show a significantreduction of these periodic mould level fluctuations.