Objective: Research suggests that the nature of combat may affect later development of posttraumatic stress disorder (PTSD) in combat veterans. Studies comparing rates of PTSD across different conflicts indicate that the use of asymmetric or guerilla-style tactics by enemy fighters may result in higher rates of PTSD among U.S. military personnel than the use of symmetric tactics, which mirror tactics used by U.S. forces. Investigations of the association between enemy combat tactics and PTSD across conflicts were limited because of cohort effects and a focus on male veterans. The current study examined rates of PTSD diagnosis in a sample of male and female veterans deployed to Operation Iraqi Freedom (OIF), a conflict with 3 distinct phases marked by varying enemy tactics. Method: Participants were 738 veterans enrolled in Project VALOR (Veterans’ After-Discharge Longitudinal Registry) who deployed once to OIF. Participants completed a clinician interview as well as self-report measures. Results: Male veterans deployed during the OIF phase marked by asymmetric tactics were more than twice as likely to be diagnosed with PTSD as those deployed during the other 2 phases, even after controlling for extent of combat exposure, demographic characteristics, and other deployment-related risk factors for PTSD. Differing rates of PTSD across the 3 OIF phases were not observed among female participants. Conclusion: The nature of combat (specifically, asymmetric enemy tactics) may be a risk factor for the development of PTSD among males. Factors other than enemy tactics may be more important to the development of PTSD among females.