Microbial contaminant degradation may either result in the utilization of the compound for growth or act as a protective mechanism against its toxicity. Bioavailability of contaminants for nutrition and toxicity has opposite consequences which may have resulted in quite different bacterial adaptation mechanisms; these may particularly interfere when a growth substrate causes toxicity at high bioavailability. Recently, it has been demonstrated that a high bioavailability of vapour-phase naphthalene (NAPH) leads to chemotactic movement of NAPH-degrading Pseudomonas putida (NAH7) G7 away from the NAPH source. To investigate the balance of toxic defence and substrate utilization, we tested the influence of the cell density on surface-associated growth of strain PpG7 at different positions in vapour-phase NAPH gradients. Controlled microcosm experiments revealed that high cell densities increased growth rates close (< 2 cm) to the NAPH source, whereas competition for NAPH decreased the growth rates at larger distances despite the high gas phase diffusivity of NAPH. At larger distance, less microbial biomass was likewise sustained by the vapour-phase NAPH. Such varying growth kinetics is explained by a combination of bioavailability restrictions and NAPH-based inhibition. To account for this balance, a novel, integrated ‘Best Equation’ describing microbial growth influenced by substrate availability and inhibition is presented.