The studies were carried out on the cotton mealybug, Phenacoccus solenopsis Tinsley (Hemiptera: Sternorrhyncha: Coccoidea:  Pseudococcidae) in Scale Insects and Mealybugs Department Laboratory, Plant Protection Research Institute, Sharkia Branch during the period extended from July to October 2015 to study the developmental stages periods of the insect under laboratory conditions 25 ± 1˚C, 65 ± 5% RH and a photoperiod 12 hrs. Three numphal instars were recorded for males and females but males had an additional stage that pupal stage. The results indicated that eggs incubation period was 1.06 days for males and females. The developmental periods for first, second and third nymphal instars, adult female longevity, life cycle and generation were 6.15, 7.26, 7.81, 18.91, 41.20 and 26.95 days, respectively. The developmental periods for first, second and third nymphal instars, pupal stage and adult male longevity were 5.91, 7.06, 6.68, 6.12 and 2.97 days, consecutively. The sex ratio was 1: 6.65 male and female, respectively. This study may be useful information for mass rearing and designing a comprehensive pest management program and prediction models for the cotton mealybug.