Kudoa septempunctatais a myxosporean parasite that infects the trunk muscles of olive flounder (Paralichthys olivaceus) and has been reported to cause foodborne illnesses in humans. However, the molecular mechanisms underlyingK. septempunctataspore toxicity remain largely unknown. In this study, the gastroenteropathy ofK. septempunctatawas examined in human colon adenocarcinoma cells as well as experimental mice inoculated with spores. We found thatK. septempunctatadecreased transepithelial resistance and disrupted epithelial tight junctions by deleting ZO-1 in Caco-2 monolayers. Additionally, serotonin (5-HT), an emetic neurotransmitter, was increased inK. septempunctata-inoculated cells. In vivo,K. septempunctataspores induced diarrhea in suckling mice (80% in ddY and 70% in ICR mice), with a minimum provocative dose of 2 × 105K.septempunctataspores. In house musk shrews,K. septempunctatainduced emesis within 1 h and induced serotonin secretion in the intestinal epithelium. In conclusion,K. septempunctatamay induce diarrhea and emesis by increasing intestinal permeability and serotonin secretion.