Two 11.8 kb non-conjugative, but mobilizable R plasmids designated pRAS3.1 and pRAS3.2 were isolated from Aeromonas salmonicida subspecies salmonicida and atypical A. salmonicida, respectively. Differences between the plasmids were of minor extent and they are considered as being variants of the same plasmid, pRAS3. The genes repA, repB, mobA, mobC, mobD, and mobE were organized similar to corresponding genes in the small, mobilizable plasmid pTF-FC2 isolated from Acidithiobacillus ferrooxidans (previously Thiobacillus ferrooxidans). The nucleotide identity between these genes from pRAS3.1 and pTF-FC2 ranged from 89.5 to 98.2%. The tetA(C), tetR(C), and approximately 960 base pairs adjacent to tetR(C) were highly similar to the nucleotide sequence in pSC101. Plasmid pRAS3 was also found in a Scottish A. salmonicida strain, and appears to be identical to the R plasmid pJA8102-2 isolated from A. salmonicida in Japan.