1. Polytomella citri encysts only when the cell-body contains considerable starch. 2. Encystment is not due to any perceptibly adverse environmental factors. 3. Optimum conditions for growth and reproduction are concomitant with maximum encystment. 4. Prevention of encystment, either by continuous transfer or by low temperature, if carried to sufficient extent, will result in morphological degeneracy and loss of the tendency to store starch and to encyst.