1) Nearly two hundred species of troglobites are known from the groundwaters of Japan. Most of these troglobiontic species, sixteen of seventy-seven genera, and what is more, four of forty-seven families are endemic to Japan. Uchidastygacaridae, Nipponacaridae, and Kantacaridae are endemic acaridan families of Japan. The coleopterous family, Phreatodytidae, is alto endemic to Japan. 2) Though studies on Protozoa, Turbellaria, Annelida, Aschelminthes, and Ostracoda, etc. remain sparse, the interstitial fauna is actively investigated recently and many specimens of Bathynellacea, Ingolfiella, Bogidiella, Microcerberus, Pseudovermis (Opisthobranchia), and Nerillidae, etc. have been collected from freshwater and marine environments. 3) None of the troglobites is known to be directly detrimental to human health and most of them have been collected from well-waters which are regarded as chemically clean in many cases, but they have also been obtained occasionally from bacteriologically contaminated well-waters. 4) Ecological and taxonomic knowledge, of even the limited amount which we possess at present, has enabled us to utilize various animals which occur in well-waters as biological indicators of well-water pollution and to have some insight as to the origin of the pollution.