Degradation of n-alkanes in diesel oil by Pseudomonas aeruginosa strain WatG (WatG) was verified in soil microcosms. The total petroleum hydrocarbon (TPH) degradation level in two bioaugmentation samples was 51% and 46% for 1 week in unsterilized and sterilized soil microcosms, respectively. The TPH degradation in the biostimulation was of control level (15%). The TPH degradation in aeration-limited samples was clearly reduced when compared with that in aeration-unlimited ones under both sterilized and unsterilized conditions. Addition of WatG into soil microcosms was accompanied by dirhamnolipid production only in the presence of diesel oil. These findings suggest that degradation of n-alkanes in diesel oil in soil microcosms would be facilitated by bioaugmentation of WatG, with production of dirhamnolipid, and also by participation of biostimulated indigenous soil bacteria.