rRNA genes are thought unlikely to be laterally transferred, because rRNA must coevolve with a large number of cellular components to form the highly sophisticated translation apparatus and perform protein synthesis. In this paper, the authors first hypothesized that lateral gene transfer (LGT) might occur to rRNA genes via replacement of gene segments encoding individual domains of rRNA: the 'simplified complexity hypothesis'. Comparative sequence analyses of the 16S and 23S rRNA genes from a large number of actinomycete species frequently identified rRNA genes containing short segments with an abnormally high number of non-random base variations. These variations were nearly always characterized by complementing covariations of several paired bases within the stem of a hairpin. The nature of these base variations is not consistent with random mutations but satisfies well the predictions of the 'simplified complexity hypothesis'. The most parsimonious explanation for this phenomenon is the lateral transfer of rRNA gene segments between different bacterial species. This mode of LGT may create mosaic rRNA genes and occur repeatedly in different regions of a gene, gradually destroying the evolutionary history recorded in the nucleotide sequence.