Previous investigations of 16S rRNA clone libraries from a wide spectrum of mainly terrestrial origin have shown the worldwide distribution of several as yet uncultivated phylogenetically deeply rooting groups of Actinobacteria. From the percentage of the occurrence of these clones it was concluded that these organisms constitute a significant part of the bacterial microflora in these habitats. Two of the clone groups, previously designated group II and group III, were shown to be phylogenetically moderately related among each other. In order to more exactly determine the abundance of a representative of group II, clone DA079, the fraction of the organism's rRNA in total extracted rRNA was determined in several neighboring samples from Drentse A grassland soil (The Netherlands). The fraction ranged from 2.6 to 9.1%, averaging 5.5%. Based upon comparison of total rRNA and strain DA079-specific rRNA it was concluded that on the average 2 x 10(6) cells/g of this organism are present in the investigated soil. Attempts to isolate members of one of the 16S rDNA clone groups of Actinobacteria were made with samples from a German peat bog, in which the organisms had been detected previously. Molecular detection of group III organisms by a nested PCR approach was possible in different cultivation media. Despite the wide spectrum of growth media employed the isolation of group III strains failed.