The mealybug Ferrisia gilli Gullan is a serious new pest of pistachios in California. It was first found near the town of Tulare in the late 1990s and has since spread to orchards in most pistachio-producing regions of the state. The seasonal phenology of F. gilli was evaluated in a commercial pistachio orchard in Tulare County during 2005 and 2006. During both seasons F. gilli overwintered as small nymphs and had three complete generations per year. Mealybug population densities were low and remained as immatures in March and April; by late May adult females formed and averaged (±SE) 1.3 ± 0.3 and 1.2 ± 0.3 per 0.75 m of sample branch in 2005 and 2006, respectively. The first in-season generation occurred from early June through mid-July, with mealybug densities ranging from 17.6 ± 5.6-26.4 ± 6.2 mealybugs per 0.75 m sample branch. The second in-season generation occurred from late July through September and had peak densities of 408.6 ± 93.9 and 182.0 ± 34.2 mealybugs per branch. In March and April mealybugs were located primarily on the buds and branch wood; in May the population was on branch wood as well as the rachis; from June through September the population was located primarily in the pistachio cluster. F. gilli's seasonal phenology described herein was used to develop a management program.