In Pseudomonas fluorescens strain CHA0, the response regulator gene gacA controls expression of extracellular enzymes and antifungal secondary metabolites, which are important for this strain's biocontrol activity in the plant rhizosphere. Two Tn5 insertion mutants of strain CHA0 that had the same pleiotropic phenotype as gacA mutants were complemented by the gacS sensor kinase gene of P. syringae pv. syringae as well as that of P. fluorescens strain Pf-5, indicating that both transposon insertions had occurred in the gacS gene of strain CHA0. This conclusion was supported by Southern hybridisation using a gacS probe from strain Pf-5. Overexpression of the wild-type gacA gene partially compensated for the gacS mutation, however, the overexpressed gacA gene was not stably maintained, suggesting that this is deleterious to the bacterium. Strain CHA0 grown to stationary phase in nutrient-rich liquid media for several days accumulated spontaneous pleiotropic mutants to levels representing 1.25% of the population; all mutants lacked key antifungal metabolites and extracellular protease. Half of 44 spontaneous mutants tested were complemented by gacS, the other half were restored by gacA. Independent point and deletion mutations arose at different sites in the gacA gene. In competition experiments with mixtures of the wild type and a gacA mutant incubated in nutrient-rich broth, the mutant population temporarily increased as the wild type decreased. In conclusion, loss of gacA function can confer a selective advantage on strain CHA0 under laboratory conditions.