The regulatory function of extracellular ATP (exATP) in bacteria is unknown, but recent studies have demonstrated exATP induced enhanced secondary metabolite production and morphological differentiation in Streptomyces coelicolor. The growth of Streptomyces coelicolor, however, was unaffected by exATP, although changes in growth are common phenotypes. To identify bacteria whose growth is altered by exATP, we measured exATP-induced population changes in fast-growing microbes and actinomycetes in compost. Compared with the water-treated control, the addition of 10 ml 100 mu M ATP to 10 g of compost enhanced the actinomycetes population by 30% and decreased fast-growing microbial numbers by 20%. Eight microbes from each group were selected from the most populated colony, based on appearance. Of the eight isolated fast-growing microbes, the 16S rRNA sequences of three isolates were similar to the plant pathogens Serratia proteamaculans and Sphingomonas melonis, and one was close to a human pathogen, Elizabethkingia meningoseptica. The growth of all fast-growing microbes was inhibited by ATP, which was confirmed in Pseudomonas syringae DC3000, a pathogenic plant bacterium. The growth of six of eight isolated actinomycetes strains, all of which were identified as close to Streptomyces neyagawaensis, was enhanced by ATP treatment. This study suggests that exATP regulates bacterial physiology and that the exATP response system is a target for the control of bacterial ecology.