11S and 18S fractions of yeast mitochondrial RNAs, isolated by electrophoresis through agarose gels, have been found by electron microscopy to contain approximately 50% circular molecules. Circles in the 11S fraction have a contour length of 0.36 +/- 0.02 micron, which is approximately equal to the length of the majority of linear molecules also present. Circles in the 18S fraction have an average length of 0.78 +/- 0.11 micron. The size distribution is broader than for the 11S fraction, and we cannot exclude the possibility that more than one size class may be present. The 11S circular RNA forms circular R loops and RNA-DNA hybrids with DNA fragments of the oxi 3 region of mtDNA, which contains the structural gene for subunit 1 of cytochrome oxidase. As judged from the electron micrographs, the complete RNA participates in hybrid formation and the sequences coding for it appear to be continuous. Both 11S and 18S circles withstand treatment with DNAase and pronase. They are not eliminated by treatment with 1 M glyoxal in 50% formamide for 1 hr at 50 degrees C. We conclude that they are covalently closed. The function of the circular RNAs is unknown. They may be active as mRNAs, storage forms, or arise in a cut-and-splice process which generates mRNAs from longer transcripts.