Protozoa found in the glacial melt waters and freshwater ponds or streams of Antarctica were investigated for their ability to tolerate freezing in their active stages. Sampling was conducted at Cape Bird in both the northern and middle colonies so as to include as wide a range of aquatic environments as possible ranging from glacial melt waters to warm eutrophic ponds. Analysis of samples found ... about 20 species of protozoa and they were cultured in the field in petri dishes with water and a piece of raw meat. A number of each species was fixed for examination with both a light microscope and a scanning electron microscope in New Zealand. Samples of all species were tested for their ability to withstand freezing, by cooling them at different rates to temperatures of between -5C and -10C. High mortality was found. These animals are exposed to low temperatures over the winter and therefore the study examined the factors which determined the occurrence of particular species of protozoa in specific localities and microenvironments. Samples were collected, dried and weighed and then rehydrated. The microfaunal content of the samples was then examined at intervals during the next 36 hours for survival. In addition, the protozoan tolerance to high temperatures was investigated.