Laboratory experiments were conducted to study the effects of soil aeration on a soil microbial community in a cambisol with a pH of 5.3 and a carbon content of 2.2%. Anaerobically incubated soil samples (7 and 14 days) which were transferred to aerobic conditions showed a rapid decline (by 30-58%) and subsequent increase in microbial biomass. The decline of microbial biomass was accompanied by an increase in specific respiration rate of microorganisms and was reduced, but not eliminated, by the addition of glucose. Carbon extraction by 0.5 M K2SO4 (C-K2SO4) was used as a measure of the release of intracellular compounds from cells damaged due to aeration. C-K2SO4 did not decrease in the soil preincubated in anaerobic conditions for 7 days and no significant changes in direct counts of bacteria were found. Thus the observed decrease in microbial biomass can be explained more likely by shrinkage than by the damage of microbial cells. When the soil was preincubated in anaerobic conditions for 14 days, the decline in microbial biomass was accompanied by an increase in C-K2SO4 indicating death of a part of the microbial community. On the other hand, a rapid decline of hot-water extractable carbohydrates which had accumulated during anaerobic conditions occurred after aeration, which indicates acceleration of substrate utilization due to oxygen supply. The changes caused by aeration indicate the existence of a transient period with depressed microbial biomass but activated metabolism before a new equilibrium was reached.