p-Toluenesulphonic acid was degraded at equal rates by two Comamonas testosteroni T-2 and L-l strains in shake flasks. However, immobilised cells of strain T-2 in a bioreactor had a degradation efficiency of up to 70% compared to 59% with strain L-l. In both open systems, initial added strains were the only p-toluenesulphonate consumers, but the composition of biofilm microbial population significantly varied.