In fresh-water lakes only a few species have proved themselves capable of inhabiting that severe and unusual habitat, the anaerobic profunal zone of the lake bottom. Although this qualitatively limited fauna is ordinarily exposed to continuously low temperatures, little or no light, a pH often falling below neutrality, relatively large amounts of free carbon dioxide, a total lack of free dissolved oxygen for months at a time, and in some instances the accumulation of certain decomposition gases, it nevertheless comprises in such situations a population of bottom-dwelling forms frequently amounting to many thousand individuals per square meter. It is the purpose of this paper to present the results from a study of the macroscopic profundal bottom fauna in which the principal emphasis has been placed on the ecological relations of the animals involved rather than on the qualitative and quantitative aspects alone, as has been so frequently done in the past. Aquatic productivity is certainly to be measured in terms of quality and quantity. However, to stop there is to leave a large and fundamentally important part in the complete problem of biolgical productivity wholly untouched. The aim throughout this investigation has been to discover not only the nature, quantity and distribution of the bottom fauna beneath the deeper waters, but having done that, to study and measure the environmental influences in an attempt to evaluate their limnological importance. During the work, the writer has become convinced of the need for great caution in pointing out any single factor as the sole determining influence in the abundance and distribution of bottom fauna.