We present the first indication of a direct relationship between a nuclear and a mitochondrial splicing system. The intron in the precursor of the large, nuclearly coded ribosomal RNA of two species of Tetrahymena possesses all the features of a class of fungal mitochondrial introns. Sequences conserved in mitochondrial introns of different fungal species are also found in the same order in these Tetrahymena nuclear introns, and the intron RNA can be folded to form a secondary structure similar to that proposed for mitochondrial introns by Davies et al. (1982). This "core" secondary structure brings the ends of the intron together. Furthermore, the first intron in the precursor of the large, nuclearly coded rRNA of Physarum polycephalum also has the characteristic conserved sequences and core RNA secondary structure. The limited sequence data available suggest that the intron in the large rRNA of chloroplasts in Chlamydomonas reinhardtii also resembles the mitochondrial introns. Tetrahymena large nuclear rRNA introns also have an internal sequence that can act as an adaptor by pairing with upstream and downstream exon sequences adjacent to the splice junctions to precisely align the splice junctions. These nuclear introns therefore fit the model of the role of intron RNA in the splicing process that was proposed by Davies et al. (1982), suggesting that the mechanisms of splicing may be very similar in these apparently diverse systems. It is therefore probable that the RNA secondary structures for which there is good evidence in the case of mitochondrial introns will be found to form the basis of active site structure and precise alignment in splicing and cyclization of the Tetrahymena intron "ribozyme".