The xanthid crab Zosimus aeneus inhabits coral reefs in the Indo-West Pacific region, along with two other species of the same family, Atergatis floridus and Platypodia granulosa. These crabs are known to contain paralytic shellfish poisoning (PSP) toxins.1,2) Furthermore, Noguchi et al.3,4) reported that A. floridus from a certain area con tained tetrodotoxin (TTX) and related substances in addi tion to PSP. The toxic components in these crabs vary in dividually and by region. In Taiwan, Zosimus zeneus is consumed by fishermen and coastal inhabitants. However, data relating to the toxicity of the crabs are currently un available for the region. In this study, the toxicity and tox ic components of Z. aeneus in Taiwan were investigated. Six and ten specimens of Zosimus zeneus were collected from Wanlitung and Hsiaoliuchiu, Pingtung Prefecture in southern Taiwan in January and April 1993, respectively. The specimens were kept alive during transportation to the laboratory and divided into appendage, cephalothorax and viscera to determine the anatomical distribution of tox icity by the PSP bioassay method.5) Following bioassay, toxic extracts (5,000 MU) were combined and filtered through a Diaflo YM-2 membrane. The filtrate (4,000 MU) was freeze-dried, dissolved in a small amount of water, ap