ABSTRACT Many recent Asian clinical Vibrio cholerae E1 Tor O1 and O139 isolates are resistant to the antibiotics sulfamethoxazole (Su), trimethoprim (Tm), chloramphenicol (Cm), and streptomycin (Sm). The corresponding resistance genes are located on large conjugative elements (SXT constins) that are integrated into prfC on the V. cholerae chromosome. We determined the DNA sequences of the antibiotic resistance genes in the SXT constin in MO10, an O139 isolate. In SXT ^MO10 , these genes are clustered within a composite transposon-like structure found near the element's 5′ end. The genes conferring resistance to Cm ( floR ), Su ( sul II), and Sm ( strA and strB ) correspond to previously described genes, whereas the gene conferring resistance to Tm, designated dfr18 , is novel. In some other O139 isolates the antibiotic resistance gene cluster was found to be deleted from the SXT-related constin. The El Tor O1 SXT constin, SXT ^ET , does not contain the same resistance genes as SXT ^MO10 . In this constin, the Tm resistance determinant was located nearly 70 kbp away from the other resistance genes and found in a novel type of integron that constitutes a fourth class of resistance integrons. These studies indicate that there is considerable flux in the antibiotic resistance genes found in the SXT family of constins and point to a model for the evolution of these related mobile elements.