We have investigated the number, the location, the orientation and the structure of the seven ori sequences present in the mitochondrial genome of a wild-type strain, A, of Saccharomyces cerevisiae. These homologous sequences are formed by three G + C-rich clusters, A, B and C, and by four A + T-rich stretches. Two of the latter, p and s, are located between clusters A and B; one, l, between clusters B and C; and one r, either immediately follows cluster C (in ori 3-7), or is separated from it by an additional A + T-rich stretch, r', (in ori 1 and ori 2). The most remarkable differences among ori sequences concern the presence of two additional G + C-rich clusters, beta and gamma, which are inserted in sequence l of ori 4 and 6 and in the middle of sequence r of ori 4, 6 and 7, respectively. Neglecting clusters beta and gamma and stretch r', the length of ori sequences is 280 +/- 1 bp, and that of the l stretch 200 +/- 1 bp. Hairpin structures can be formed by the whole A-B region, by clusters beta and gamma, and (in ori 2-6) by a short AT sequence, lp, immediately preceding cluster beta. An overall tertiary folding of ori sequences can be obtained. Some structural features of ori sequences are shared by the origins of replication of the heavy strands of the mitochondrial genomes of mammalian cells.