Abstract Exposure of a Tetrahymena culture to numerous heat shocks of the type used to synchronize cell divisions results in cells having a volume up to 10 times that of untreated cells. Besides, a few percent of the cells are hereditarily modified and give rise to lines of monstrous polymorphic cells. To facilitate production and isolation of monstrous cells a heat shock program combined with a selection procedure and a shortened genetic dilution test has been tested. All treatments known to produce the modified cells initially result in cellular growth without accompanying divisions. It is shown that increase in cellular volume as produced by the heat shocks is not sufficient to induce the modifications. It is suggested that multiplication of cortical cytoplasmic structures or morphogenetic reference points may be essential. Some similarities between the hereditarily modified Tetrahymena and cancer cells are mentioned together with the possibility that hereditary changes may sometimes be induced by conditions generally considered unable to do so.