Clinical tests revealed that the extracts of Gelidium amansii (Gelidiaceae) had a anthelmintic action and further examinations were made on the anthelmintic components of this seaweed. This anthelmintic principle is absorbed on alumina and eluted from it by alkali solution. The active principle is absorbed on activated carbon from aqueous extract and eluted from it by methanol and it is not adsorbed on Amberite IR-120(H-form). This anthelmintic effective fraction was prepared by the use of this properties. Action of the active principle of Gelidium amansii was examined pharmacologically. The active principle of Gelidium amansii was found to decrease the tensity, tonus and mobility of Eisenia foetida(Savigny) nerve muscles. The active principle of this effective fraction was submitted to paper chromatography and spots to ninhydrin were detected at Rf; 0.30-0.31(yellow), 0.26(violet), 0.2(violet), 0.14-0.13(violet), 0.9(orange) and 0.04(violet).