The effect of soil aggregation on denitrification has been studied in different ways: artificial or natural aggregates individually analysed or comparison of denitrifying activity of different size classes of aggregates. However, until now, no work has been conducted to evaluate the role of denitrification in the microbial colonisation of soil aggregates. Over a one-month period, we examined on remoulded nonsterile soil aggregates the survival of inoculated wild-type Pseudomonas strain and of its corresponding isogenic Tn5 mutant (Nir(-)) lacking the ability to synthesize the dissimilative nitrite-reductase. Simultaneously, the evolution of the O-2 repartition inside the soil aggregates was assessed by the use of microelectrodes. The inner and outer portions were roughly anoxic and oxic, respectively, during the entire experiment while the intermediate portion showed fluctuating aeration conditions. The values of the Nir(-) to wild-type + Nir(-) ratio were found in the following order: inner < intermediate < outer portion, demonstrating that the nitrite-reductase may provide a competitive advantage to the Pseudomonas strain to colonise the centre of soil aggregates. However, a clear differentiation between inner and outer aggregates portions was not observed with the indigenous microflora (denitrifiers-to-total heterotrophs, biomass or physiological abilities).