Summary1TheCitrusaphis reproduces parthenogenetically continuously throughout the year; sexual reproduction has not been observed.2The most favourable temperature for development and reproduction of the aphis is between 22 and 25° C. Between 18 and 22° C. the rate of reproduction and development is also suflicient to bring about heavy infestation.3The temperature below 18° C. is unfavourable for considerable increase of the population of this hisect. At 7° C. reproduction is barely taking place.4At 30–32° C. the mortality of the nymphs is almost 100%. At 34–35° C. reproduction ceases, and at 36° C. adult insects die within a short period.5The annual fluctuations of the population of the insect based on field observations are described.6Food as a factor in the increase of the population of the aphis is discussed.7The effect of the annual fluctuations of the temperature in Palestine upon the population of the aphis is pointed out.8The economic status of the insect is