Plasmids of Streptococcus thermophilus were previously classified, based on DNA homology, into at least four groups (A-D). Here, we report the characterization of plasmids of group B and D. The sequence analysis of pSMQ173b (group D) indicates that this plasmid contains 4449 bp, five open reading frames (ORFs) and replicates via the rolling-circle mechanism of the pGI3 family. The plasmid pSMQ308 (group B) contains 8144 bp and six ORFs. Two ORFs likely encode a primase/helicase and an integrase. Northern blot experiments demonstrate that these two orfs are transcribed within the three strains containing plasmids of group B. Two-dimensional agarose gel electrophoresis shows that pSMQ308 replicates via a theta mechanism. To our knowledge, this is the first report of a plasmid replicating via a theta mode in S. thermophilus. Finally, a classification of 20 sequenced S. thermophilus plasmids into six groups based on their mode of replication is proposed.