Abstract: The relationship between activity of soil microorganisms and soil matric suction was studied. The concept of bubbling suction, which is the soil matric suction at which air permeability of soils becomes non-zero, was applied. The objective of this experiment was to test the hypothesis that highest microbial activity in soil occurs when soil matric suction is slightly larger than the bubbling suction because both water and oxygen are abundant. In all the three soils tested, the highest microbial respiration was observed at soil matric suctions just larger than the bubbling suction. The microbial activity at small matric suction reached the highest value slower than that around bubbling suction.