The Aphelinid parasitoid Eretmocerus mundus Mercet is considered the most effective parasitoid against Bemr'sia argentr'folii Bellows & Perring. a serious pest in greenhouses and outdoor crops, in Egypt. Studies were carried out to determine some biological characters of Er. mundus under two temperature regimes 25 and 30 °C and ?5% RH. The life cycle of the parasitoid shortened at 30 °C (22.31189 days). become longer (218152 days) at 25 °C. Also, the female longevity lasted 9.21-2.45 and 12.55134 days at 30 °C and 25 °C. respectively. At 30 °C. female progeny was 58514.88 progeny, while it was 44.89i1.32 at 25 °C. In addition. the statistical analyses revealed that the parasiloid life cycle. female longevity and progeny rats were varied significantly according to the temperature regime used. Constant number of Er. mundus was released against various numbers of its host. Higher parasitism percentage (87.5%) was recorded with the ratio of 1 parasiloid: 5 B. argentr'folr'i nymphs. The parasitism percentages decreased when host numbers increased. When parasitoids released at ratios of 1: 10. 1: 15 and 1: 20, the parasitism percentages decreased to 65.96%. 55.5 and 40.4%. respectively. This means that increasing host numbers from 5 to 20 B. argentifolr'r‘ nymphs. nymphal mortality decreased 6.5 times in comparison with 1: 5 ratio.