In Egypt, the clearwing moth S. myopaeformis (Lepidoptera: Aegeriidae) is a serious pest on apple trees. The relationship between weather factors (of temperature and relative humidity) and the population fluctuation was quantitatively calculated during six successive years (from 1997 to 2002 separately, and 1997-2002 together) in apple orchards at Qalubia governorate. Trials were conducted to determine the correlations between the main weather factors and moths activity as well as using the day-degree method for predicting the peak emergence period of adult moths, i.e. to asses prediction formula through which population fluctuation could be expected. R-square values of each single weather factor indicated that daily maximum (Xl) and minimum temperature (X2) significantly affected S. myopaefomils population fluctuation, showing 0.461-0.958 for (X1) and 0.607-0.904 for (X2) and were included in . selection of suitable statistical models used. Statistical combined models ((X1X2), (X1X12), (X2)22), (X1X22), (X12X2), (X12)(22) and (X1X2X12X22)} were used In assessing the prediction formula. The effective weather factor was the daily maximum temperature (Xl) rather than minimum temperature (X2). Prediction calculations were based on the linear regression formula described by Bishop (1969)