Cultures of Tetrahymena pyrvriformis, T. thermophila and T. pigrnentosa have been studied with regard to growth rates in shaken and unshaken flasks. In the standard medium, a minimum doubling time of 170 min was obtained for T. pyriformis at 28" C in the unshaken cultures. If the depth of the medium was less than 1 cm, the gyratoric shaking increased the doubling time to 340 min. The effect of shaking could be reduced by the addition of dextrane. Cells subjected to shaking were observed in different media and at different growth temperatures. Ifcultures were inoculated with lo4 cells.ml-l or more, the effect of shaking was absent. However, with inoculates of lo3 or 10' cells.ml-', the doubling times for T. pyrzformis increased to 240 and 275 min. respectively. Periods of 2 min shaking followed by rest for 60 rnin could not induce an effect. 5-91, 7-17-91; accepted 9-6-91 Received 4-

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