Abstract: Caloglossa leprieurii(Mont.) J. Ag., under the vernacular name of "Zhegocai", was investigated for its actions against ascaris with the preparation of its decoctum, because this marine alga has long been used as an anthelmintic drug in Chinese folk medicine. In in vitro observations, it was demonstrated that the solution of Caloglossa leprieurii was fatal to earth-worms even in low concentration which often caused active movements of the worms before they were killed. It was also found that on both Ascaris Lumbricoides and Ascaris Suum, they were first stimulated and then depressed by this drug in laboratory studies. Clinically, it was observed that among 4 groups of patients, two of which were adults receiving a dose of 20-30 grams, and another two were children receiving a dose of 8-15 grams of Caloglossa leprieurii, the use of the drug resulted in over 80% of ascarids elimination in three groups, and nearly 50% in one group. Some side cffects of this drug, such as nausea, mild abdominal pain, and dizziness were occasionally noted. Thirty mice receiving the solution of Caloglossa leprieurii in single oral dose of 0.4, 1.0, or 2.0 gm/kg were tested for toxicity, but no reactions were observed during a period of 3-day observation.