Two plasmids, pTF-FC2 and pTC-F14, that belong to the IncQ-like plasmid family were isolated from two related bacteria, Acidithiobacillus ferrooxidans and Acidithiobacillus caldus, respectively. The backbone regions of the two plasmids share a sufficiently high amount of homology to indicate that they must have originated from the same ancestral plasmid. Although some of their replication proteins could complement each other, the plasmids have evolved sufficiently for their replicons to have become compatible. This compatibility has occurred by changes in the iteron sequence, RepC (iteron binding protein) specificity and the regulation properties of the RepB primase. Two of the five mobilization genes have remained highly conserved, whereas the other three genes appear to have evolved such that each plasmid is mobilized most efficiently by a different self-transmissible plasmid. Plasmids pTF-FC2 and pTC-F14 do not appear to compete at the level of mobilization. The antitoxins of the toxin-antitoxin (TA) plasmid stability systems were partly able to neutralize the toxins of the other plasmid and also to partly cross-regulate the TA systems of the other plasmid with the antitoxin of pTF-FC2 being the most effective cross-regulator. Other aspects of the evolution of the two plasmids are described and the danger of making the assumption that incompatibly of IncQ-like plasmids is a reflection of the degree of relatedness of two plasmids is discussed.