Genetic investigations of plant pathogenic bacteria date back to the late 1950s with the early reports claiming transformation in Xanthomonas (41, 42) and Agrobacterium (120, 121). Acquisition of virulence in planta by nonpathogen ic agrobacteria foIlowing mixed inoculation with crown-gaIl-inducing strains of Agrobacterium was first reported in 1969 (122) and later confirmed (125, 126). Subsequent developments in the early 1970s include: (a) the demonstra tion of F' and R plasmid transmission to Erwinia and Pseudomonas via conjugation (19, 140, 199), (b) the demonstration that plasmids are virulence elements in Agrobacterium (263, 271), (c) the detection of Ti-plasmid sequ ences in crown-gall-cell DNA (24), and (d) the advent of recombinant DNA techniques. These studies raised hopes that gene transfer and recombination systems could be developed in phytopathogenic bacteria along the lines of classical bacterial genetics. They also served as a basis for the development of a v<ifiety of methods and tools for in vivo and in vitro genetic manipulation in these organisms. The demonstration of Ti plasmids as virulence elements fueled an unprecedented research effort that by a decade's end had culminated in the development of the most promising method for plant genetic engineering. Recombinant DNA techniques provide important new tools for basic and applied genetic research in plant pathology. The genetics of phytopathogenic bacteria were reviewed in 1979 (141). Several chapters of a book (177) published in 1982, and a numbcr of reviews and book chapters on the molecular biology of Ti plasmids (12, 92, 94, 111, 112, 163, 164, 179, 180, 228, 254, 264), on the genetics of Erwinia and