The response of fresh-water protozoan communities exposed to both severe acute temperature shocks as well as small gradual long-term increases are discussed. The former experiments were carried out in plastic troughs with a constant flow of unfiltered lake water. Severe acute shocks (some to nearly 50 C) resulted in a marked reduction in number of species present. However, recovery was quite rapid (a matter of a few days) once the temperature stress ceased. Observations of the effects of small gradual long-term increases were made on the protozoan communities of the Savannah and Potomac rivers each of which received heated waste water discharges. Each of these studies covered a period in excess of nine years and observations are still being made. At the time the paper was prepared there was no evidence that indicated the protozoan communities of these rivers had been degraded by the small gradual temperature increases resulting from the discharge of heated waste waters. However, there is evidence that competitive exclusion of algal species by other more tolerant algal species may cause qualitative shifts in the community structure which may be undesirable. It is probable that similar shifts occur in protozoan communities–a factor which should be considered in future studies.