A 4.0 Kb (2.64 Mdal) plasmid was isolated from a fowl cholera strain of Pasteurella multocida (the Larsen strain) by alkaline lysis and cesium chloride purification. The plasmid, designated pLAR-1, was characterized in terms of its size and restriction sites. The restriction patterns produced by fourteen endonucleases were used to generate a restriction map. Five restriction enzymes cleaved the plasmid at multiple sites. Two enzymes, Bgl II and Sal I had unique sites on pLAR-1. Twelve of the fifty six strains of P. multocida surveyed contained plasmids of different sizes which hybridized to pLAR-1. Strains containing homologous plasmids were variable in serotype, dermonecrotoxin production, and origin (both in terms of the host and locale). pLAR-1 did not encode any of the enzymes necessary for the biochemical pathways contained within the API-20E strip or siderophore production. pLAR-1 was cloned into the BamH I site of pBR322. Resultant clones were approximately 8.363 Kb in length, ampicillin resistant and tetracycline sensitive. The pLAR-1