Discophrya collini subjected to high levels of feeding onParamecium caudatum developed giant forms in culture. These take several forms: a single enlarged cell, a giant with attached normal cells or attached giants with normals. All the cells possess functional tentacles. The giant cells show qualitative and quantitative macronuclear changes and an abnormally thickened epiplasm containing membraneous profiles and other aberrant structures. These cells contain kinetosome fields and brood pouches identical to those found during normal swarmer production. It is suggested that the giant complexes are formed by the normal production of swarmers but a failure in their release from the adult, perhaps attributable to the abnormal epiplasm, results in their subsequent metamorphosisin situ. The abnormal epiplasm could be produced by the deposition of myelin body food residues from the cytoplasm. The initial induction of gigantism itself may be related to disruption of the normal growth-division cycle similar to that experienced during natural senescence. Possible mechanisms of this disruption and differences with other suctoria are discussed.