Further attempts were made to identify the toxins of the horseshoe crab Carcinoscorpius rotundicauda which causes sporadically food poisonings in Thailand. Toxic C. rotundicauda eggs were crushed and extracted with water repeatedly. After being defatted with dichloromethane, the combined extracts were ultrafiltered to cut off more than 1,000 dalton-substances. Toxic fractions were obtained from the filtrate by column chromatography on Bio-Gel P-2 and Bio-Rex 70. Cellulose acetate strip electrophoresis, TLC, HPLC, and other instrumental analyses demonstrated that the horseshoe crab toxin was essentially composed of tetrodotoxin and related substances, along with paralytic shellfish poisons as minor components.