Summary. Three plasmids which each dclermincd Ihc same extensive anlibiotic resistance phenotype, including resistance to gentamicin. and which had been recovered from organisms involved in an outhreiik oigentamicin-resistant infections in Melhourne hospitals in [1975][1976], were analysed by fingerprinting with the restriction endonucteases Eco Rl, Hind HI and Psl I. In each case the fingerprints for all three plasmids were identical, despite the tact that each plasmid originated in a different host species at ditTerent hospitals. This observation confirms our earlier hypothesis (Davey and Pittard, 1977) that the piasmids mediating gentamicin resistance in the outbreak were the closely related descendants of one ancestraUlncL) plasmid.