In our previous study, Pseudomonas fluorescens Pf0-1L, harbouring the IncP-7 carbazole-degradative plasmid pCAR1 : : rfp, was shown to be undetectable within 5 days post-inoculation in carbazole-contaminated artificial freshwater microcosms containing several plasmid-free bacteria in addition to Pf0-1L(pCAR1 : : rfp). Fourteen days after the inoculation, carbazole degraders become detectable. Here, we revealed that these isolates were not pCAR1 transconjugants, but Pf0-1L(pCAR1 : : rfp) mutants, based on RFLP and BOX-A1R-based repetitive extragenic palindromic-PCR (BOX-PCR) analysis. Notably, the mutants displayed more rapid initiation of carbazole degradation than the parent strain Pf0-1L(pCAR1 : : rfp). The mutants were unable to degrade anthranilate due to a 163 bp deletion in the antA gene, which was overcome by their transformation with a wild-type antABC-expressing plasmid. Quantitative RT-PCR analysis indicated that the transcriptional induction of carbazole-, anthranilate- and catechol-degradative genes was comparable in both parent and mutant strains. The deletion mutants became dominant in the artificial water microcosm. The mutation caused anthranilate to accumulate instead of catechol, a toxic compound for the parent strain, and may be beneficial to host survival in artificial microcosms.