Primary mucinous carcinoma of the fallopian tube is extremely rare. We report the detailed characterization of a mucinous carcinoma arising in the fimbrial end of the fallopian tube in a 74-yr-old woman. The patient presented with recurrent urinary tract infection and urinary tract obstruction secondary to a large right ovarian mass. She had an appendicectomy as an 11 yr old. Serum CA-125 was raised at 239 U/mL. Computed tomographic scans showed bilateral, cystic ovarian tumors but no other intra-abdominal abnormality. The patient underwent total abdominal hysterectomy, bilateral salpingo-oophorectomy, and omental biopsy. Microscopy showed mucinous carcinoma arising from the left tubal fimbriae, in association with mucinous metaplasia over the uninvolved fimbrial folds. There was no mucinous metaplasia in the contralateral fallopian tube, endometrial lining, cervix, or on the uterine serosal surface. A mucinous borderline tumor of gastrointestinal subtype was identified in the left ovary. The right ovary contained a benign mucinous cystadenoma of mixed gastrointestinal and endocervical-like/Müllerian subtype. The fallopian tube tumor expressed CK7, claudin 18, and MUC6, but not CK20, CDX2, CEA, pyloric gland mucin (recognized by HIK1083), ER, or vimentin. The immunolabelling pattern for p53 was wild-type, and p16 expression was nonblock. The metaplastic mucinous tubal epithelium also marked for CK7, CK20, CDX2, and CEA but had mutation-type p53 labelling (p53 null), a low Ki-67 index, and was immunopositive for HIK1083, MUC6, and claudin 18. This is the first detailed characterization of a primary mucinous fallopian tube carcinoma and the adjacent metaplastic mucinous epithelium, and confirms it to be of gastric type.