Research evaluating human immunodeficiency virus (HIV) behavioral interventions among U.S. military personnel has been lacking. In this study, a behavioral intervention to prevent HIV and sexually transmitted diseases (STDs) was implemented in a sample of Marine security guards. Participants were assessed before and after a three-session intervention on a measure of STDs/HIV knowledge and a number of psychosocial scales. The results indicated that STDs/HIV knowledge was significantly greater after the intervention. Significant pre-test vs. post-test differences were also found on the Social Norms, Behavioral Intentions, Attitudes toward Condoms, and Self-Efficacy/Impulse Control scales. On Social Norms and Behavioral Intentions, the differences were as expected: subjects perceived greater social norms supporting condom use and had stronger intentions to practice safe sex after the intervention than they had before. On Attitudes toward Condoms and Self-Efficacy/Impulse Control, the differences were not in the expected direction. Psychosocial factors associated with self-reported condom use were also identified.