During batch growth of Alcaligenes eutrophus on various aromatic compounds in the presence of acetate, several distinct behaviour patterns were observed, The utilization of substrates of the meta pathway (phenol or p-cresol) was inhibited by acetate. When the aromatic was a substrate of the p-hydroxybenzoate branch of the ortho pathway, growth was mixotrophic, i.e. both substrates were consumed simultaneously. For the substrates of the gentisate pathway or the benzoate branch of the ortho pathway, substrate preference was governed by growth performance, Aromatic compounds enabling growth rate and yields higher than those obtained on acetate alone (i.e. benzoate, benzaldehyde, m-hydroxybenzoate and gentisate) inhibited acetate utilization, while acetate was the substrate consumed preferentially in mixtures containing aromatic compounds supporting only slow growth (i.e. benzoyl formate and 4-fluorobenzoate).