Here we report a case of Strongyloides stercoralis infection of the gastric and pancolonic mucosa in a 79-year-old female with a monoclonal gammopathy of undetermined significance. Endoscopic biopsies were performed in gastric antrum, cecum, distal ascending colon, and hepatic flexure of the colon. On microscopic examination, there were many adult worms, larvae and eggs in the gastric and colonic mucosa. Worms, larvae, and eggs were located in the crypts and within the lumen of the crypts. The body wall of the adult worm was composed of cuticle and a weak muscle layer. A routine stool examination failed to detect larvae or ova. Based on the histopathologic examination, these parasites were confirmed as S. stercoralis.