The alignment of gene sequences coding for A. nidulans mitochondrial L-rRNA and E. coli 23S rRNA indicates a strong conservation of primary and potential secondary structure of both rRNA molecules, except that homologies to the 5'-terminal 5.8S-like region and the 3'-terminal 4.5S-like region of bacterial rRNA are not detectable on mtDNA. The structural organization of the A. nidulans mt L-rRNA gene corresponds to that of yeast omega + strains: both genes are interrupted by a large intron sequence (1678 and 1143 bp, respectively) and by another smaller insert (91 and 66 bp) at homologous positions within domain V. An evolutionary tree derived from conserved L-rRNA gene sequences of yeast nuclei, E. coli, maize chloroplasts and six mitochondrial species exhibits a common root of organelle and bacterial sequences separating early from the nuclear branch.