More than 70 conjugative R plasmids were isolated from wild-type [Escherichia coli] strains originating mainly in Southeast Europe and identified as incompatible with the reference plasmid R387 of the incompatibility group K. These plasmids, governing different resistance patterns, were characterized as [phage] P1 cotransducible species of between 50 and 60 Mdaltons. In contrast to the genetic similarity between all these plasmids and the reference type R387, their DNA revealed different digestion patterns after EcoRI treatment, although a number of common fragments could be identified. The molecular and genetic properties of these plasmid species demonstrate a phylogenetic relatedness.