Theoretical considerations of denitrification within soil aggregates lead to the conclusion that this process should increase with increasing aggregate size. The objective of this paper was to verify this statement for homogeneous, spherical soil aggregates. The artificial aggregates were formed from clay (48% 2 µm) out of the Ah horizon of a Vertisol (Tröbersdorf) with the following diameters < 2, 6.0, 10.0, 15.0, and 23.0 mm. Half of the aggregates were unamended and half NO3 (100 mg NO₃^–-N kg^–1) + C (400 mg of glucose C kg^–1) amended and then incubated at a constant soil water tension of 0.5 kPa for 4 days at 28 ± 1°C in an atmosphere containing 0.03 l l^–1 acetylene. The denitrification rate (as measured by N₂O evolution) and respiration rate (O₂ uptake) for all the aggregate sizes as well as the distribution of aerobic and anaerobic bacteria and denitrifiers in the biggest aggregates were studied. It was found that denitrification rate increased and respiration rate decreased with increasing aggregate size, both in amended and unamended soil aggregates. The number of denitrifiers and the total number of anaerobic bacteria increased while numbers of anaerobic bacteria tended to decrease in the aggregate centre. The theoretical prediction of the dependence of denitrification rate on aggregate size was confirmed.