Efficiency of degree-day of temperature on the activity of Bemi­sia tabaci (Genn.) was worked out, in the present study, over two toma­to growing seasons (Nili plantations of 1996 and 1997). Results of this study infer that the degree-day model and the population of B.tabaci (Germ.) adults are highly correlated, and therefore, the degree-day mod­el index (Heat units) could be taken as an indication on B.tabaci (Genn.) abundance on the tomato fields. The simple regression value (b) indicate that for every 0.5-0.6°C increasing in the degree-days, on an average, there is a 1% infestation in the population density of Etabaci adults and so on. Such results are very important for managing this pest.