A field account of the behavior and ecology of the gregarious and corticolous psocopteran Cerastipsocus sivorii is presented. The study was conducted from February to November 2003 on the campus of the Universidade Estadual de Campinas, state of São Paulo, southeastern Brazil. There was a strong positive correlation between the relative abundance of host tree species and their respective frequency of occupation by C. sivorii, suggesting that trees were used according to their availability in the study site. The phenology of C. sivorii is seasonal, with nymphs peaking in May and October, and teneral adults peaking in February, June, and October. The factors determining the variation in population density in psocopterans are poorly understood, but our data show that climatic variables, such as rainfall and temperature do not influence the phenology of C. sivorii. The individuals of C. sivorii remain together through the entire nymphal phase, resting, moving on the tree surface (mainly on bark, but occasionally on leaf petioles) and grazing in groups. Teneral adults within an aggregation usually dispersed a few days after molting. Nearly 50% of the aggregations had up to 90 individuals, but large groups presenting 240 individuals or more were also frequent, comprising 10% of all aggregations found in the field. When a moving aggregation encountered another one, they occasionally either interchanged individuals or merged into a single large group. More rarely, large aggregations divided into two groups. We suggest that gregariousness in C. sivorii is a behavioral strategy that confers protection against predation and reduces body water loss.