The current study prospectively examines the predictive relationship of trait dissociation, assessed during academy training, to PTSD symptoms assessed at 12 months of active police duty in relatively young and healthy police academy recruits (N = 180). The roles of pre-academy trauma exposure, exposure to life-threatening critical incidents during police duty, and peritraumatic dissociation at the time of the officer's worst critical incident were also examined. Utilizing path analytic techniques, greater trait dissociation, assessed during academy training, was predictive of both peritraumatic dissociation, and PTSD symptoms assessed at 12 months of police service. Moreover, after accounting for trait dissociation and peritraumatic dissociation, the relationship of previous trauma to later PTSD symptoms was no longer significant, demonstrating that the effect of previous trauma on later vulnerability to PTSD symptoms in this sample may be mediated by both trait and peritraumatic dissociation.