Summary The freshwater hymenostome Lembadion bullinum exists in two forms: “normal” cells and “giants”. The general morphology of both morphotypes was studied, biometrical data for both morphs are presented and compared with each other. Beside a sharp increase in cell length and width, giants are characterized by their larger oral region, by an increased number of meridional kineties, by a larger macronucleus and in certain stocks by the occurrence of additional micronuclei. It is shown that the majority of giants present in a culture originates in binary fission of already transformed cells. The ex novo transformation of normal cells into giants regularly occurred in starved cultures as a consequence of cannibalism. At higher cell densities (≥ 250 cells/ml) relatively more individuals transformed into giants than at low cell densities (≤ 50 cells/ml). There is no evidence for any soluble or cell bound “giant inducing factor” in this species.