Summary Pseudomonas putida BN210, carrying the self‐ transferable clc ‐element encoding degradation of 3‐chlorobenzoate on the chromosome, was used as inoculum in different membrane biofilm reactors treating 3‐chlorobenzoate‐contaminated model wastewater. Analysis of the bacterial population in the effluent and in the biofilm showed the loss of BN210 beyond detection from the reactors and the appearance of several novel 3‐chlo...