A bacterial co-culture, ECO3, constituted by a polychlorobiphenyl degrading bacterium, Pseudomonas sp. strain CPE1, and two chlorobenzoic acid degrading bacteria, could grow on Aroclor 1221 (75 mg/L) as the sole carbon source without accumulating chlorinated aromatic metabolites in the medium; 44.5% of the Aroclor 1221 organic chlorine was detected as chloride ion in the medium after 115 h of incubation in batch condition. When glass beads (diameter = 3 mm, 30% w/v) or Triton X-100 (0.066% v/v) were added to ECO3 cultures, average dechlorination percentages were 80% and 89.5%, respectively, after the same incubation time. These percentages were significantly higher than those previously observed with the only polychlorobiphenyl degrading member of ECO3, CPE1 strain, in the same culture conditions. This result can be ascribed to the capability of the ECO3 chlorobenzoic acid degrading bacteria of completely mineralizing the chlorinated benzoic acids produced during the Aroclor 1221 degradation. The depletion of these intermediates seems to prevent toxic or inhibitory effects on the bacteria thus permitting a larger Aroclor 1221 metabolization.