1. Phenothiazine does not possess ascaricidal effect in vitro but was found to effect paralysis of ascaris in a motionless state, in a comparatively dilute solution. These observations have prompted further detailed anthelmintic studies. 2. A total of up to 12 gm. of phenothiazine was administered to 625 ascariasis patients for 1-5 days in various doses and their anthelmintic effect was examined in detail by the egg-counting and also worm-counting methods. It was thereby found that the chemical possessed anthelmintic effect against ascaris and pin-worm but in the dose used, the effect was not superior to that of 0.1 gm. (0.05gm.×2)of santonine against the former. 3. ·Oral administration of 0.2 gm./kgm. of phenothiazine to rabbits and dogs for over 5 consecutive days caused temporary decrease of erythrocytes and hemoglobin content, and transitory increase of leucocytes and reticulocytes. 4. Effect of phenothiazone in causing motionless state of ascaris in vitro and anthelmintic effect in man were far weaker than those of phenothiazine.