A remarkable cell shape change was observed inBacillus subtilisstrain 168 under microculture conditions on CI agar medium (Spizizen's minimal medium supplemented with a trace amount of yeast extract and Casamino acids). Cells cultured under a cover glass changed in form from rod-shaped to spherical, large and irregular shapes that closely resembled L-form cells. The cell shape change was observed only with CI medium, not with Spizizen's minimum medium alone or other rich media. The whole-cell protein profile of cells grown under cover glass and cells grown on CI agar plates differed in several respects. Tandem mass analysis of nine gel bands which differed in protein expression between the two conditions showed that proteins related to nitrate respiration and fermentation were expressed in the shape-changed cells grown under cover glass. The cell shape change of CI cultures was repressed when excess KNO3was added to the medium. Whole-cell protein analysis of the normal rod-shaped cells grown with 0·1 % KNO3and the shape-changed cells grown without KNO3revealed that the expression of the branched-chainα-keto acid dehydrogenase complex (coded by thebfmBgene locus) was elevated in the shape-changed cells. Inactivation of thebfmBlocus resulted in the repression of cell shape change, and cells in whichbfmBexpression was induced by IPTG did show changes in shape. Transmission electron microscopy of ultrathin sections demonstrated that the shape-changed cells had thin walls, and plasmolysis of cells fixed with a solution including 0·1 M sucrose was observed. Clarifying the mechanism of thinning of the cell wall may lead to the development of a new type of cell wall biosynthetic inhibitor.