A defined mixed culture of Bacillus subtilis and Stenotrophamonas maltophilia was used to accomplish the partial biodegradation of the ate-dye p-aminoazobenzene (pAAB). Kinetic experiments were conducted, under aerobic conditions, to study the mineralization of p-aminoazobenzene by the above-defined mixed culture, under aerobic conditions. The combination of two previously developed models, (Zissi et al., 1997), which describes pAAB biodegradation by Bacillus subtilis into aniline and p-phenylenediamine, and (Zissi and Lyberatos, 1999), which describes aniline biodegradation by Stenotrophomonas maltophilia, is shown to predict well the anticipated mixed culture growth and partial biodegradation of pAAB. In previous work (Zissi et al., 1997) it was observed that pphenylenediamine was unstable during the experiments therefore the fate of p-phenylenediamine was not studied. The overall kinetic model of the defined mixed culture was then used to study the behavior of the mixed culture system in a range of operating conditions in the chemostat. The partial degradation of pAAB (regarding one of the two products, aniline) was described by an interaction between the two bacteria with competitive and commensalistic elements. The two bacteria are shown to coexist in a CSTR for some ranges of the operating variables. (C) 2001 John Wiley & Sons, Inc.