Bacterial plasmids originating in a wide range of genera are being studied from a variety of perspectives in hundreds of laboratories around the globe. These elements are well known for carrying "special" genes that confer important survival properties, frequently neces sary under atypical conditions. Classic examples of plasmid-borne genes are those provid ing bacterial resistance to toxic substances such as antibiotics, metal ions, and bacte riophage. Often included are those determining bacteriocins, which may give the bacterium an advantage in a highly competitive environment. Genes...