The genome of the soil bacteriumPseudomonas putidaKT2440 bears two virtually identicalarsRBCHoperons putatively encoding resistance to inorganic arsenic species. Single and double chromosomal deletions in each of thesearsclusters of this bacterium were tested for arsenic sensitivity and found that the contribution of each operon to the resistance to the metalloid was not additive, as either cluster sufficed to endow cells with high-level resistance. However, otherwise identical traits linked to each of thearssites diverged when temperature was decreased. Growth of the various mutants at 15°C (instead of the standard 30°C forP. putida) uncovered thatars2 affords a much higher resistance to As (III) than thears1counterpart. Reverse transcription polymerase chain reaction ofarsB1andarsB2genes as well aslacZfusions to thePars1andPars2promoters traced the difference to variations in transcription of the corresponding gene sets at each temperature. Functional redundancy may thus be selected as a stable condition – rather than just as transient state – if it affords one key activity to be expressed under a wider range of physicochemical settings. This seems to provide a straightforward solution to regulatory problems in environmental bacteria that thrive under changing scenarios.