* Author for correspondence Summary

and T. malaccensis have been studied with regard to control of cell volume and cellular dry weight. Cell volume was measured on cells suspended in 0-9 % sodium chloride + 0-1 % sodium azide using a Multisizer cell counter (Coulter). Tetrahymena were grown at different temperatures and under various up- or downshift conditions. In all cases the changes in cell volume are paralleled by changes in cellular dry weight. The volume and the dry weight of a Tetrahymena cell are determined by the par-ticular medium and the growth temperature. Large cells are seen in concentrated media and at low growth temperatures resulting in cell volumes up to 17000/im3, whereas starving cells decrease gradu-ally towards 700 firn3 or even smaller sizes. It is proposed that lag phase observed at up- and down-shift is to a large extent due to the necessary adjustment of the cell volume to the new con-ditions. Key words: cell volume, cell dry weight, Tetrahymena