Effects of D-alanine and Mitomycin-C on cell morphology of Agrobacterium tumefaciens strains F-010 (wild-type strain) and F-502 (thermoconditional morphology mutant strain) were investigated at 37° (a non-permissive temperature). D-Alanine induced following morphological changes: Normal cell form, rod shape at 27° (a permissive temperature) in both F-010 and F-502 strains, is altered by D-alanine to an abnormal cell form, ovoid (or spherical) and tadpole-like shapes, respectively, at 37°. In the presence of Mitomycin-C, individual cells of F-010 strain showed a successive morphological alteration of elongation and branching. When both D-alanine and Mitomycin-C were added to the culture of F-010 strain., form of the cells altered to a tadpole-like shape. These observations might be explained as follows: (i) D-Alanine suppresses an activity to maintain the normal cell form (rod form) in newly elongated part of the cell; (ii) Mitomycin-C is a potent inhibitor of septum formation and subsequently induces branch formation at one polar zone of a cell according to the "maximal growth rate model"; and (iii) the functional unit, which is sensitive to Mitomycin-C, in the wild-type strain, is identical to the functional unit, which is temperature-sensitive, in the mutant strain.