Osmoregulated periplasmic glucans (OPGs) ofRhodobacter sphaeroidesare anionic cyclic molecules that accumulate in large amounts in the periplasmic space in response to low osmolarity of the medium. Their anionic character is pro-vided by the substitution of the glucosidic backbone by succinyl residues. A wild-type strain was subject to trans-poson mutagenesis, and putative mutant clones were screened for changes in OPGs by thin layer chromatogra-phy.