Qualitative and quantitative investigations of the invertebrate fauna of aquatic and terrestrial habitats have commonly noted the omnipresence and the consistently large number of individuals of the Nematoda. They occur as ecto- and endoparasites of many other invertebrates, vertebrates and plants, and as free-living forms. The parasitic nematodes have been studied more intensively, especially those parasitizing the vertebrates, because of their interest and significance to man, and because, in general, they are larger in size than the non-parasitic forms. In the past, qualitative studies of the parasitic forms occupied a major portion of nematological research, but lately the emphasis has shifted to the investigation of the more quantitative aspects of their biochemical and, to a certain extent, their biophysical relationships. In recent years considerable progress has been made in the study of the life history, distribution, and ecology of the plant-parasitic and associated terrestrial forms. However, in contrast, these aspects of the biology of the free-living nematodes, especially the aquatic forms, still remain largely unstudied. Even though free-living, aquatic nematodes are known to be abundant and ubiquitous throughout the biosphere, they have not received a proportionate share of attention. The present investigation was undertaken with these facts in mind. It was deemed advisable from the beginning to concentrate attention on one species of aquatic, free-living nematode, because of the complexity of the taxonomy of these worms. Dorylaimus stagnalis was chosen for this study because it is: 1) cosmopolitan, 2) relatively large, 3) abundant in a lake (Douglas) whose biota and physico-chemical conditions have been studied in considerable detail, and because the taxonomic identity and distinctness of this species is generally understood and accepted.