Mutant Agrobacterium tumefaciens strain A66 is shown to differ from its wild-type progenitor (strain A6) by a spontaneous 2.7 kb DNA insert into the T-DNA region of its Ti plasmid. Tobacco stems transformed by A66 exhibit an attenuated response characterized by slow growth and shoot proliferation. Clonal analysis demonstrates that this response is due to an alteration in the growth and regenerative potential of transformed cells, rather than to variation in the frequency of fully autonomous cells within the primary tumor. Cloned A66 transformed tobacco cells exhibit an auxin requirement for growth that can be overcome by shoot proliferation. Other host species, however, may complement the A66 mutation yielding fully auxin-independent tumors when transformed by this bacterium.