Trematode larvae of the family Accacoeliidae were obtained from Flaccisagitta enflata collected in the Mexican Caribbean Sea during February, March, May, and August 1991. Two parasites were found in the coeloms of 2 chaetognaths of 14,583 examined (prevalence = 0.014%; intensity = 1). The parasites have 5 or 6 branches on each side of the anterior intestinal ceca and 2 deeply stained zones, 1 internal on the posterior edge of the acetabulum and the other a glandular structure around the cloacal pore in the posterior part of the body. Two tegumental folds are present on the anterior and posterior parts of the parasite. These trematode larvae, their host, and their occurrence in the Mexican Caribbean Sea are reported for the first time.