Intestinal secretory IgA (sIgA) response or lack of response among adults in Mexico with diarrhea was used as an indicator of enteropathogenicity of Aeromonas species and Plesiomonas shigelloides. sIgA was extracted from stool specimens obtained at day of presentation and 5 days later. Total sIgA was standardized, and specific sIgA titer against the organism being shed by each patient was determined. Western blotting was used to determine which microbial antigens elicited an intestinal sIgA response. Of 12 subjects shedding Aeromonas sobria or Aeromonas hydrophila, 11 had a fourfold or greater sIgA titer rise against the infecting strain. Western blotting showed that somatic lipopolysaccharides were the immunodominant antigens. No sIgA titer rises were detected among two patients shedding Aeromonas caviae or in 14 shedding P. shigelloides. This study provides further evidence of the significance of A. sobria and A. hydrophila as pathogens in acute diarrhea but raises additional questions about the role of P. shigelloides, at least in US adults with travelers' diarrhea.