To clarify the effect of calcium ions on the growth and transformation of S. platensis, the alga was cultured in calcium ion-deficient medium. The growth of neither spiral nor linear forms of S. platensis was not affected by calcium ions. However, when cultivated in calcium ion-deficient medium, the spiral form transformed into the linear form. Linear algae that had been cultured for a long period in the calcium ion-deficient medium did not revert to the spiral form on cultivation in the calcium-containing SOT medium.