The internal pO2 of soybean (Glycine max Merr.) nodules was measured with oxygen microelectrodes. For nodules in air at 23°, the pO2 decreased sharply across the nodule cortex, and was too low to measure in the central tissue. At 1° in 1.0 atm O2, the pO2 in the central tissue was measurable, and was approximately uniform from the center to the edge of the central tissue. This uniformity was probably due to the intercellular air spaces of the central tissue, since infiltrating the spaces with water substantially decreased the pO2 in the central tissue. The results strongly suggest that most of the resistance to O2 diffusion into the nodule occurs within the cortex.