Pre- and postdeployment serum samples obtained from U.S. marines in Operations Desert Shield and Desert Storm were tested for antibodies to Shigella sonnei. High predeployment levels of immunoglobulin A (IgA) and/or IgG antibodies to S. sonnei lipopolysaccharide antigen in serum and seroconversions were accompanied by higher IgA and/or IgG antibody response to Shigella invasion plasmids (Ipa). The results suggest exposure to S. sonnei in predeployment troops and frequent exposure to this bacterial agent during deployment.