A highly conserved cryptic plasmid is present in Chlamydia trachomatis yet naturally occurring plasmid-deficient isolates are very rare. This paper describes the isolation and characterization of a plasmid-deficient strain of C. muridarum, using novobiocin as a curing agent. Plasmid-deficient derivatives of C. muridarum strain Nigg were generated at high efficiencies (4-30%). Phenotypic characterization revealed that the cured derivative was unable to accumulate glycogen within intracytoplasmic inclusions. In addition, this strain formed small plaques at a reduced efficiency compared to the wild-type parent.