Degradation of 4-chloroaniline in the presence of aniline by a microbial community in a laboratory-scale biofilm reactor was evaluated. The starter inoculum was isolated and reconstructed from a percolating column enrichment of Indonesian agricultural soil. The capacity to mineralise and detoxify 4-chloroaniline in the presence of aniline was demonstrated by the biofilm reactor when operated at high hydraulic retention time (HRT; 0.87 h). At low HRT (0.23 h and 0.39 h) 4-chlorocatechol accumulated in the effluent, accompanied by a decrease in dechlorination and detoxification. When returned to high HRT (2.14 h), the accumulation of 4-chlorocatechol stopped and the extent of dechlorination and detoxification increased. Bacteria other than the original inoculum appeared in the reactor when the operating mode was switched from closed cycle to open cycle. One of these bacteria, identified as Pseudomonas putida RI by partial 16S rDNA sequencing, subsequently dominated the reactor at every HRT imposed. PCR-based single-strand conformational polymorphism of 16 s rDNA and traditional cultivation procedures indicated that the bacterial composition in the reactor shifted in response to applied HRT. The relationship between the bacterial abundance and the degradation capacity of the reactor is discussed.