We have studied the seasonal changes in size of Amoeba spumosa in both flat land and mountain fresh water. In flat land (locality river Brembo near Bergamo), the greatest increase in size of Amoebae was found during the month of May (increase 239.2% from February to May) and another increase, not so marked, during September. In mountain water (locality brook of Tonale Pass, at 1880 m) in the Amoebae was found a curve with only one maximum peak in July (increase 287.1% from February to July). The correlation size-temperature, discovered in these two places, is once more proved in the correlation temperature-size observed in other different Italian areas. The greatest dimensions of Amoebae which coincide with an optimum temperature are presumably the result of the interaction of many favourable factors mainly the increase of the producers. The remarkable increase in size, over a period of several generations of Amoebae treated with RNP extracted from protist Chlamidomonas, brought to same an increase of over 25% in comparison to the largest found in nature. RNP-treated Amoebae appeared different from these latter because of a smaller nucleo-plasmic ratio.