The cotton whiteXy, Bemisia tabaci (Genn.) (Hom., Aleyrodidae) is increasingly a very important pest on many vegetables, Weld crops and ornamental plants. Therefore, controlling of this pest is still needed especially under glasshouse conditions. The specialist whiteXies' predator, Serangium parcesetosum Sicard (Col., Coccinellidae) appears to have a great potential for the cotton whiteXy control. In this study, the dynamic changes in B. tabaci populations in glasshouse cabins in response to S. parcesetosum were monitored. B. tabaci were introduced to cotton plants in three cabins in average of 50 adults per plant. One and two weeks later, adult females and males of S. parcesetosum were introduced at a rate of one female and one male per plant in the Wrst and second cabins, respectively. The third cabin was considered as a control. The results showed that the mean number of whiteXies in the control cabin was found signiWcantly higher than that of either when S. parcesetosum was introduced 1 or 2 weeks after the infestation with the whiteXy. Also, the mean number of B. tabaci was signiWcantly higher when the predator was introduced 2 weeks rather than 1 week after B. tabaci infestation. The maximum mean weekly number of whiteXies/plant was 192.3 in the second week, whereas it was 294.6 in the third week and 1136.4 in the Wfth week, in Wrst, second and control cabins, respectively. In the last experimental week, the mean weekly numbers were 74.7, 122.9 and 684.7 whiteXies/plant in the three cabins, respectively. S. parcesetosum has been successfully fed, reproduced and established its population on B. tabaci on cotton plants. The mean weekly number of the predatory individuals increased gradually with the progress of the experimental time. The results demonstrated that the maximum reduction percentage in B. tabaci population was 90.7 and 86.5% in the Wfth week after B. tabaci infestation, when the predator was introduced 1 and 2 weeks after the infestation with the whiteXies, respectively. Nevertheless, it is speculated that an earlier release of S. parcesetosum would be more eVective in the biological control of B. tabaci.