Two Small cryptic plasmids designated pONE429 and pONE430 were isolated from a rumen bacterium, Selenomonas ruminantium S20. The complete sequence of pONE429 was 2100 bp and contained one open reading frame (ORF) of 201 amino acids. The sequence of pONE430 had 1527 bp and one ORF of 171 amino acids with the similarity of replication protein (Rep protein) of pOM1, pSN2, and pIM13 isolated from Butyrivibrio fibrisolvens, Staphylococcus aureus, and Bacillus subtilis, respectively. In these plasmids, the upstream nucleotide sequence of Rep protein had the conserved nucleotides which could be double-strand origin (DSO) of rolling circle replication (RCR) mechanism. The plasmids of pONE429, pONE430, pJJMI, pJDB21, and pS23 were isolated from S. ruminantium strains and had similar regions that were located within a <450-bp nucleotide. These similar regions may be the location that was recognized by the host strain, S. ruminantium.