A large plasmid enables its host Agrobacterium tumefaciens to cause tumorous condition in a wide variety of dicotyledonous plants[see Ooms et al. Gene 14:33--50 (1981) )). The location and role of chromosomal genes in this phenomenon are not known. As the first stage in studying this aspect, a project was initiated to investigate the chromosomal genetics of the bacterium. R68.45, a P group plasmid, was chosen as a transmission agent. After a preliminary assessment it was decided to use C58 as a standard strain to carry out the mapping. The plasmid itself, as judged by the presence of antibiotic markers, appears to be stable in A. tumefaciens; its ability to promote chromosomal mobilisation, however, remains only in 60--80% transconjugants. Good Agrobacterium donors are capable of transferring chromosomal genes at a frequency varying between 10(-5) to 10(-6) per recipient. The recombinants are stable even under non-selective conditions. A linear linkage map consisting of 16 markers was built using coinheritance frequencies obtained from 21 four-point crosses.