Biologists have developed a sensor that employs one-celled protozoa to detect toxins in water sources. The new technique works by introducing protozoa into small chambers with water samples taken from municipal, industrial, or military water sources and comparing them to control samples. Any alteration of the protozoa’s swimming mechanics is a sign that water conditions have changed and chemical or biological contaminants — pesticides, industrial chemicals, or biological warfare agents — may be present.