Workplace bullying refers to prolonged exposure to frequent hostile behaviors at work, which can lead to severe stress reactions. Research in this area has not revealed a clear picture on how bullying escalates in organizations. Drawing on recent developments in work stress theory, this study tested a comprehensive model of bullying in which work environmental and personality factors were hypothesized to act as antecedents of bullying and post-traumatic stress symptoms as an outcome. Structural equation modeling on data provided by 609 public sector employees in Italy showed that job demands (workload and role conflict) and job resources (decision authority, co-worker support and salary/promotion prospects) were related to bullying over and above neuroticism, and that bullying mediated the relationship between job demands and PTSD symptoms. Evidence also emerged for a buffering effect of job resources on the job demands-bullying relationship. Overall results are compatible with a view of bullying as a strain phenomenon, initiated by both work environmental and personality factors.