The present investigation was carried out through two successive years (2005/2006 and 2006/2007) in Qalubyia governorate to determine some ecological aspects of the green shield scale, Pulvinaria psidii (Mask.) (Hemiptera: Coccidae) on mulberry trees. The obtained results showed that, P. psidii had two main periods of seasonal activity occurred in autumn (one generation) and summer (two overlapping generations) per each year. The 1^st period of seasonal activity lasted for 4 months, started from mid September to early January and had one generation, peaked in mid November or early December with mean numbers of 26.5-69.6 insect/leaf. The 2^nd period of seasonal activity extended from early May to mid August with duration of 4 months and had two overlapping generations. The 1^st generation peaked in early June and the 2^nd in mid July / early August in both studied years. The population abundance of P. psidii considerably decreased from mid January to mid April (in both studied years) forming a long depressive period due to falling of the infested leaves in winter and occurrence of unfavorable environmental conditions as well as emergence of new leaves in spring. The combined effect of the tested weather factors (daily mean max. & min. temperatures and %R.H.) on the seasonal activity of P. psidii ranged 65.7-74.7% in autumn and 68.9-89.4% in summer seasons, respectively.