Giardia lamblia cysts were harvested from Mongolian gerbils and exposed to free chlorine in buffered water at pH 5, 7, and 9 at 15 C. The contact times required to obtained a 2-log reduction in cyst survival (i.e., a 99% kill) were interpolated from survival curves generated at fixed concentrations of chlorine in the range of 0.25 to about 16 mg/liter. Concentration-time (C times t') products for 99% inactivation ranged for about 120 to nearly 1,500 mg times min/liter. These values are higher than those reported previously for free chlorine using G. lamblia cysts from infected humans. The cysts isolated from gerbils, as with other Giardia cysts, were unusually sensitive to chlorine in alkaline solutions. (Copyright (c) 1989, American Society for Microbiology.)