Studies have not examined the factor structure or measurement invariance of posttraumatic stress disorder (PTSD) symptomatology using population-based data. Confirmatory factor analysis of the PTSD Checklist-Civilian Version (PCL-C) was conducted in a representative sample of U.S. active duty military personnel (N = 15,593). Consistent with prior research, a 4-factor model consisting of reexperiencing, avoidance, emotional numbing, and arousal factors was superior to four alternative models. Measurement invariance was found for factor loadings, but not observed item intercepts when comparing personnel with and without a recent deployment (<or=12 months). Findings indicate differences in the level of observed responses across deployment subgroups that exceed what would be expected for individuals with similar PCL latent factors scores. Implications of results are discussed.