The prevalence, diversity and linkage of pTA-related plasmid rep and mob genes in 18 plasmids from three closely related species of Bacillus from diverse geographical locations have been investigated using PCR. pTA-related rep and mob sequences were both amplified from 13 of the 18 plasmids. For one plasmid, pBM9, only a pTA-related mob gene was amplified, whilst only a pTA-related rep gene was amplified from the Bacillus licheniformis plasmid pBL2. No products were amplified for either gene from two other B. licheniformis plasmids or from a larger 16-kb Bacillus subtilis plasmid, pBS6. Whilst simple gene detection suggests close linkage of both pTA-related rep and mob genes, on most of these plasmids, sequence analysis of amplified genes revealed more complex linkage relationships varying with geographical origin, plasmid size and bacterial host.