The nucleotide sequence of the smallest plasmid of Erwinia stewartii SW2 was determined. This plasmid, pSW100 (4272 bp), consists of a 702-bp region homologous to the origins of replication of plasmids p15A, ColE1, and ColA. Plasmid pSW100 also contains sequences homologous to the bom region and mobCABD genes of ColE1, except that a single-base deletion in mobA has been detected. This deletion did not affect the mobilization ability of pSW100 by an endogenous conjugative plasmid of E. stewartii SW2, pDC250. Plasmid pSW100 also has a 1596-bp open reading frame with five 132-bp perfect repeats. However, this open reading frame and the repeats are not required for plasmid replication. Furthermore, curing of pSW100 did not cause any phenotypic change, suggesting that this plasmid is not essential for the survival of the organism.