A mixed culture of compatible hydrocarbonoclastic and diazotrophic bacteria, each at a density of 10(8) organisms cm(-3), was developed for optimised bioremediation of crude oil-contaminated soil. The hydrocarbonoclastic bacterium, Pseudomonas sp and the diazotroph, Azotobacter vinelandii, were both isolated from a previously crude oil-contaminated soil and thereafter modelled as a unit of mutualistic consortium in situ. Stabilisation of the consortium and hence the optimised bioremediation process occurred when the bacterial growth attained a pseudo-steady state condition. This was considered to be as a result of a symbiotic association between A vinelandii and the Pseudomonas sp in which A vinelandii produced the required concentration of fixed nitrogen compounds required for the growth of the Pseudomonas sp. Enhancement in biodegradation, due to stimulated growth of Pseudomonas sp and co-metabolic activity of A vinelandii, was mathematically evaluated as the difference in the specific growth rates (mu) between the consortium Pseudomonas sp/A vinelandii and Pseudomonas sp alone. The proportion of petroleum hydrocarbons degraded by the consortium from the contaminated soil ranged between 66.83 and 69.6% as compared with that of a pure culture of Pseudomonas sp (23.2-44.45%). Hence, beyond their role in biological nitrogen fixation, diazotrophs may be used to contribute to bioremediation of crude oil-contaminated land. (C) 2004 Society of Chemical Industry.