The association of a 20-MDa plasmid with nalidixic acid resistant (Nalr) strains ofShigella dysenteriae1 has been examined. The plasmid, which is readily transferable, does not itself code for nalidixic acid resistance but offers a survival advantage to its host under nalidixic acid stress. The plasmid-containing cultures ofS.dysenteriae1 produced Nalrmutantsin vitroat a frequency 1000-fold higher than their plasmidless parent strains, after two exposures to nalidixic acid. Using a similar procedure, mutants resistant to other antibiotics such as chloramphenicol, tetracycline, or ciprofloxacin could not be isolated. The genome ofS.dysenteriae1 appears to carry a heavy load of the insertion sequence IS1. The propensity of the plasmid-containing strains to readily mutate to nalidixic acid resistance and its possible relevance to the observed association of the plasmid with Nalrclinical isolates is discussed.Key words:Shigella, nalidixic acid, plasmid, shigellosis.