Protozoan communities inhabiting polyurethane substrates suspended in Douglas Lake, Michigan were transferred into glass jars and exposed to various concentrations of free chlorine. These communities were then examined to determine which species survived exposure. Communities exposed to free chlorine three times in a two hour period exhibited a significant decrease in number of species (relative to controls) at concentrations above 1.15 ppm. Free chlorine concentrations above 0.66 ppm administered every 20 minutes over the two hours produced significant decreases in number of species. Community diversity, determined by the Wilhm and Dorris diversity index (d) was also signficantly reduced by three exposures to concentrations above 1.15 ppm. Exposure every 20 minutes produced signficant reductions in diversity at five of the seven exposure concentrations between 0.58 ppm and 2.85 ppm. Certain species appeared more tolerant of chlorine stress than others. Individuals of these species constituted a larger percentage of the communities’ total numbers as chlorine stress increased. Preliminary results indicate that increased frequency of exposure lowers the concentration which produces a signficant reduction in diversity.