In the first half of this century, little was known of the groundwater animals of Japan. In 1916, Prof. Ijima and Dr. Kaburaki gave a description of a hypogean planarian as the first groundwater animal of Japan. Following this description, eleven species were recorded from wells and caves during the thirty-five years up to 1950. Thus, only twelve species of animals were registered as groundwater animals of Japan in the first four decades of this century. Since 1950, a number of groundwater animals have been collected and described from caves and wells of Japan, and the present list of the troglobites from the groundwaters of Japan comprises almost two hundred species which are classified in eight phyla, thirteen classes, eighteen orders, forty-seven families, and at least seventy-seven genera. However, numerous specimens of Oligochaeta, Ostracoda, Cyclopoida, and particularly those of microorganisms such as Protozoa, Hydra, Nematoda, and Rotatoria, etc. still remain undescribed. Because sufficient numbers of mature specimens in satisfactory condition are not available for study, some material of Nemertinea, Archiannelida, Turbellaria, Dytiscidae, Phreatodytidae, etc. are also left undescribed. In recent years, a remarkable decrease in the number of wells available for the collection of material has forced some speleobiologists to divert their activities to the field of the interstitial fauna of rivers and seashores. Besides the troglobites listed in Table I, various groups of troglophiles and trogloxenes have been obtained from the ground waters of Japan. Some of them are significant as a biological indicator of well-water pollution upon which I shall comment later on. Among the aquatic troglobites of Japan, one of the noteworthy groups may