AbstractA brief review of cytological studies of bacterial cell division is followed by a description of preliminary investigations of the chemical nature of factors which, in complex media, influence the division process. In these, magnesium is shown to be essential for the normal cell division of both Gram positive and Gram negative rod‐shaped eubacteria, but in significantly different amounts.In simple synthetic media, magnesium is also essential for the synthesis of bacterial protoplasm, and in the complete absence of this ion, eubacteria fail to grow. The magnesium requirements of Gram positive organisms are about ten times as great as the requirements of Gram negatives. This difference is attributed to the fact that the former also incorporate magnesium into their structure.The significance of these findings and the observed differences in cell division of the cocci and rod‐shaped organisms arc discussed.