Seasonal population activity of the whitefly, Bemisia tabaci (Genn.) in cotton Giza 86 was studied during 1999 cotton season. The experiment was carried out at Sakha district, Kafr El-Sheikh Governorate, from January to early of November. The occurrence of B.tabaci lasted about 15-17 weeks from the beginnning of Sowing. This pest had different numbers of population peaks, showing 5 peaks during the season of activity. Immature stages of B.tabaci occurred in high numbers on cotton leaves during the growing period and 3-4 peaks of population were noticed. An assessment of occurrence and abundance of annual broods was approximated. Concerning mid to late season; the population of whitefly adults showed a fluctuation of four peaks on July, August, September and October representing by 320, 850, 760 and 1850 individuals /100 leaves, respectively.  For nymphs, four peaks were also observed on July, September and October representing by 300, 56, 42 and 16 individuals /100 leaves, respectively.