Diurnal shifts in the predominant species of heterotrophic bacteria in a freshwater river have been detected by numerical taxonomy testing procedures. Owing to the modifications in the database which were required to accommodate the range of species in a natural system, it was necessary to substantiate the genetic homology of the clustering procedure. DNA hybridization of a cluster ofAeromonas hydrophilastrains indicated that the cluster was 88.6% genetically homologous. The numerical methods grouped strains by physiological characteristics and this phenotypic similarity did not always coincide with the genetic relatedness of the strains. However, the diurnal changes in the predominant bacterial types can be demonstrated effectively by the database.