The purpose of these experiments was to simulate the passage of water containing protozoans through the condensers of a steam electric station in order to determine the effects upon colonization of downstream communities. For preliminary tests an apparatus consisting of a test cell constructed of acrylic plastic with a small resistance heater controlled by a variable transformer was developed to deliver a heat shock to protozoans while they were being observed through a microscope. Microthermocouples were used to measure the temperature of the fluid in the cell. Response to rapid increase in temperature was determined for the protozoans Euglena gracilis, Spirostomum ambiguum, Colpidium colpoda, Stentor coeruleus, Tetrahymena pyriformis, and Paramecium multimicronucleatum. Response to several constant temperatures was determined for E. gracilis, S. ambiguum, and C. colpoda. In another experiment, water was pumped from an artificial stream through a copper coil immersed in a hot water bath, to produce a 16 - 26°C increase in temperature. The water then flowed in the same manner into a cold water bath cooling it back to its original temperature. The water was then directed into three plexiglass troughs 79 cm. long, 5 cm. wide, and 7.5 cm. high, before it fell over an end plate 4 cm. high back into the artificial stream. Two control troughs were maintained in an identical manner except for the heat shock. The five communities were sampled weekly and records of diversity (i.e. number of species) in each community and estimates of population density of each species were made. The experiment ran for a period of seven weeks. No significant differences in diversity were observed between control and experimental communities. More subtle differences were noticed, however, which indicated that the heat shock may alter the degree to which incoming species can successfully colonize downstream habitats. / Master of Science