A plasmid, designated pFS1, has been identified for the first time in a strain of Bacillus polymyxa. The plasmid is 16 kb in length. It is sensitive to digestion by DNAase I, restriction endonucleases and exonuclease III, but resistant to RNAase A and λ-exonuclease. A restriction map of the plasmid has been constructed, demonstrating its linear nature. The linear DNA molecules of pFS1 are attached at their 5' ends to a protein which can be cleaved by Pronase. No homology was detected between pFS1 and chromosomal DNAs from a number of different B. polymyxa strains. Also, hybridization between pFS1 and total DNA from B. polymyxa SCE2 plasmid-cured derivatives does not occur. The plasmid seems to be cryptic since B. polymyxa strains harbouring pFS1 and the cured derivatives have the same phenotype.