The largest plasmid in the strain 39R861, which is used as a plasmid size standard, was recovered by conjugation and sequenced to determine its exact size. Plasmid p39R861-4 transferred at high frequency. Although reported to be the A/C1 plasmid RA1, p39R861-4 is a 155794-bp Type 2 A/C2 plasmid, in which a 39-kb segment, derived from RA1 that includes a relative of the RA1 resistance island, replaces 26.5 kb of the Type 2 backbone. p39R861-4 includes a single copy of IS10 and two resistance islands with a CR2-sul2 region in each of them. The 84 kb of backbone between the resistance islands is inverted relative to other known A/C plasmids and this inversion has arisen through recombination between the CR2-sul2 regions that are inversely oriented. The two resistance islands present before this inversion occurred were one related to but longer than that found in RA1, and one that is a form of the ARI-B island and identical to ARI-B in the A/C2 plasmid R55. They contain genes conferring resistance to tetracycline (tetA(D)), sulfonamides (sul2), and florfenicol and chloramphenicol (floR). The tet(D) determinant is flanked by two IS26 in a transposon-like structure named Tntet(D). Both resistance islands contain remnants of the two ends of the integrative element GIsul2, consistent with the sul2 gene being mobilized by GIsul2 rather than by CR2.