Haemophihis somnus is an opportunistic bacterial pathogen capable of causing pneumonia, septicemia, and other systemic infections in bovines. An H. somnus isolate from bovine abortion (strain 649) was found to carry a similar to 1.3 kb plasmid (pHS649) that contained partial homology to two previously sequenced Haemophilus/Histophilus plasmids by BLAST analyses. Sequence analysis of pHS649 identified a putative RepA protein with 48% similarity to the RepA protein of Escherichia coli plasmid pKL1. A similar to 5 kb plasmid (pHs129) from H. somnus preputial isolate 129Pt was also sequenced and found to encode two copies of a putative RepB protein. Whereas pHS649 stably replicated in E coli DH5 alpha, pHS129 did not. Genetic relatedness and possible replication mechanisms of these plasmids are described. Published by Elsevier Inc.