We have shown that 2.8 and 3.1 pm circular DNA molecules, previously reported to be present in Saccharomyces cerevisiae and Torulopsis glabrata respectively, contain sequences hybridizing to cytoplasmic ribosomal RNAs.In S. cerevisiae the 2.8 pm circular DNA appears to be identical to the rDNA repeating unit from nuclear DNA, both in length (approximately 9000 base pairs) and in the location of the 25, 18 and 5.8S rRNA sequences on the large HindIII fragment (6500 bp) and the presence of the 5S rRNA sequence on the small HindIII fragment. In S. cerevisiae the 4-5 copies of the 2.8 pm circular DNA molecules per cell, which have an extra-nuclear location, do not appear to be essential for cell viability as in one strain they were undetectable.