From a group of naturally occurring antibiotic resistance plasmids, a number of plasmids were identified which were incompatible with members of incompatibility group P and also incompatibility group I alpha or I gamma. These plasmids also exhibited strong entry exclusion with members of group P only and showed a host range which resembled that of plasmids of group I rather than those of group P. Segregants of a number of these plasmids appeared to have lost some of the incompatability and/or surface exclusion functions. Studies of nucleic acid homology indicated that these plasmids were very similar to one another. They exhibited 15 to 20% nucleic acid homology with representatives of the I alpha and I gamma groups, yet showed less than 2% homology with the group P plasmid RP4.