This research explored the changes in genetic diversity and spatial distribution of microbial communities in association with the changes in phenol concentration during a bioremediation process. Results using the traditional plate count method indicated an increase of average bacteria densities in groundwater from 10(4) to 10(7) CFU ml(-1) initially to 10(7) to 109 CFU ml(-1) after remediation. The diversity and stability of phenol-degrading bacterial communities were investigated by using single-strand-conformation polymorphism (SSCP) genetic profile analysis of 16S rDNA fragments amplified from groundwater samples. The molecular data showed a high degree of genetic similarity between communities from certain monitoring wells during the early phases of remediation, probably due to similar initial physical conditions among wells. Molecular signatures of several cultivated phenol-degrading bacterial strains could be seen in most groundwater profiles throughout the study period, suggesting that these strains were indigenous to the study site. It was also observed that the species diversity of these microbial communities increased as the phenol levels in the groundwater decreased during the 9-month study period, and recovered to the pre-treatment levels after the remediation program was completed. (c) 2006 Elsevier B.V. All rights reserved.