Field studies were conducted in southeastern Thailand on the biology of ANOPHELES BALABACENSIS Baisas, 1936, and its relationship to malarial transmission in a hyperendemic area. In recent years A. balabacensis has been found to be an important vector of human malaria in the forested hills of Thailand and adjacent countries of SE Asia. It also appears to be the most important vector of malaria in regions where drug-resistant falciparum strains have been detected. Emphasis was placed on determination of feeding habits, and the time and season of maximum abundance. A. balabacensis was found most abundantly in the latter part of the monsoon season (Sept.–Nov.); it congregated near humans in early evening, but did not feed in large numbers until after midnight. The species was found to be exophilic and exophagic, and none could be found away from the jungle during daylight hours. Gland infection rate was over 8% during the period of peak abundance, and several of our team contracted falciparum malaria which was refractory to treatment by synthetic drugs such as chloroquine. Data on larval breeding sites were accumulated, and comments made on methods for control of malaria in the study area.