Incubation of thermosensitive dna mutants of Bacillus subtilis at the nonpermissive temperature results, at the cellular level, in the appearance of swellings. The study of one particular type of swelling, named "terminal balloon", reveals that the occurrence of the latter was correlated with completion of rounds of DNA replication. Morphological and autoradiographic observations reveal that (a) cell wall consists of two layers, (b) the outer layer splits at a fixed distance from the cell pole and allows the formation of a balloon contained within a single wall layer and (c) the bulk of active synthesis of cell wall is localized in the balloon area leading to the formation of a near spherical monolayer. Implications of these results for cell wall morphogenesis are discussed.