Among the marine toxins relevant for human intoxication, tetrodotoxin (TTX) has been known as one of the most prejudicial. Puffer fish were originally thought to be the only animal from which TTX could be isolated. Subsequent works determined that TTX also existed in a range of marine organisms of different phylogenic classes and a very few amphibians. The production of TTX is generally accepted to be associated with bacteria. This paper presents the current evidences for the distribution of TTX and its analogs in several vertebrates and invertebrates, along with the probable origin of TTX.