Study of the cell wall growth in Bacillus megaterium by pulse-labeling a DAP- Lys- mutant with tritiated diaminopimelic acid (DAP) had revealed the presence of intracytoplasmic radioactivity. The nature of this radioactivity was studied on one hand by autoradiographic analysis of bacteria treated in different ways and on the other hand by chromatography of the radioactive compounds extracted with boiling water. It is shown that cytoplasmic radioactivity corresponds neither to free DAP nor to DAP metabolized into lysine, but to murein precursors. Autoradiographic analysis of bacteria in which all murein precursors were removed gives exactly the same cell wall growth pattern as the one previously obtained for untreated bacteria. It can be concluded that, in B. megaterium, cell wall elongation occurs by diffuse intercalation of newly synthesized murein along the cylindrical part of the cell wall and that only cross wall formation occurs in a precise growth zone.