The influence of human gastric juice with various degrees of acidity to hookworm larvae was systematically observed for a long period of time. The results are as follows. 1) At 37°C human gastric juice was denatured so that it was impossible to make such a study at this temperature. 2) Even with the addition of streptmycine and penicillin the denaturation of gastric juice at 37°C could not be controlled, but rather accerelated. 3) At the temperature of refrigerated room the lethality of the larvae of Anchylostoma caninum in gastric juice was in proportion to the length of time and to its acidity, but was extremely low. 4) Thus it was concluded that the human gastric juice even with very high acidity cannot prevent the oral infection of the matured hookworm larvae. In other words, oral susceptibility of human to hookworm is not influenced by gastric juice. 5) At a refrigerated-room temperature the ecdysis rate of the larvae of Anchylostoma caninum in gastric juice increased in inverse proportion to the acidity.