The design and operation of rotating-disk mechanical foam-breakers (MFRDs) fitted to tower fermenters, i.e. bubble columns (BCs), treating various foaming liquids were studied. The foam-breaking behaviour of MFRDs fitted to BCs and the foaming behaviour of BCs were investigated respectively from the changes in the required critical disk rotational speed Nc and those in liquid hold-up in the foam, øL. The effects of operating conditions on Nc and øL were evaluated quantitatively. On the basis of their results, the empirical equations for predicting Nc and øL in foam-breaking regions, useful for the design and operation of MFRDs fitted to BCs, were obtained. The level of the required foam-breaking power Pkc of MFRDs was also clarified in relation to the level of øL which reflects the foaming intensity of BCs and is related to the difficulty of foam-breaking.