Previous estimates of the depth of oxygen penetration into bacterial colonies were made after measuring actual and potential respiration rates of whole colonies, or by calculation from kinetic values determined from the growth of bacteria in liquid culture. This paper reports the use of microelectrodes to measure oxygen penetration directly. Oxygen became undetectable 25-30 microns below the surface of a 120 microns deep, 18 h colony of Bacillus cereus. The colony was grown on a nutrient-rich agar medium incubated at 30 degrees C in a water-saturated atmosphere.