The caligid copepod Pseudocaligus fugu, a common parasite on the body surface of both toxic (Takifugu pardalis) and cultured, non-toxic (Takifugu rubripes) puffer fishes, was isolated and analyzed for the presence of tetrodotoxin (TTX) using a monoclonal anti-TTX antibody-based immunohistochemical technique. Histological sections of female P. fugu from Takifugu pardalis explicitly revealed that positive brown staining was visible in regions of gut and appendages, and also in the general body tissues from the prosome to the urosome. It is absent from the epicuticle, ovary, oviduct, uterus and egg sacs, where clear negative blue color reaction was obtained. In contrast, the caligids on cultured, non-toxic Takifugu rubripes, considered as negative control, had no sign of TTX. The results indicate that there is no vertical transmission of TTX in the parasitic caligids, which could acquire TTX by feeding on the toxic mucus and skin tissues of host puffer fish.