We have previously proposed that fluctuations in Ca 2+ levels should play an important role in bacteria as in eukaryotes in regulating cell cycle events (Norris et al., J. Theor. Biol. 134 (1998) 341–350). This proposal implied the presence of Ca 2+ uptake systems in bacteria, cell cycle mutants simultaneously defective in Ca 2+-homeostasis, and perturbation of cell cycle processes when cellular Ca 2+ levels are depleted. We review the properties of new cell cycle mutants in E. coli and B. subtilis resistant to inhibitors of calmodulin, PKC or Ca 2+-channels; the evidence for Ca 2+-binding proteins including Acp and FtsZ; and Ca 2+-transporters. In addition, the effects of EGTA and verapamil (a Ca 2+ channel inhibitor) on growth, protein synthesis and cell cycle events in E. coli are described. We also describe new measurements of free Ca 2+-levels, using aequorin, in E. coli. Several new cell cycle mutants were obtained using this approach, affecting either initiation of DNA replication or in particular cell division at non-permissive temperature. Several of the mutants were also hypersensitive to EGTA and or Ca 2+. However, none of the mutants apparently involved direct alteration of a drug target and surprisingly in some cases involved specific tRNAs or a tRNA synthetase. The results also indicate that the expression of several genes in E. coli may be regulated by Ca 2+. Cell division in particular appears very sensitive to the level of cell Ca 2+, with the frequency of division clearly reduced by EGTA and by verapamil. However, whilst the effect of EGTA was clearly correlated with depletion of cellular Ca 2+ including free Ca 2+, this was not the case with verapamil which appears to change membrane fluidity and the consequent activity of membrane proteins. Measurement of free Ca 2+ in living cells indicated levels of 200–300 nM, tightly regulated in wild type cells in exponential phase, somewhat less so in stationary phase, with apparently La 2+-sensitive PHB-polyphosphate complexes involved in Ca 2+ influx. The evidence reviewed increasingly supports a role for Ca 2+ in cellular processes in bacteria, however, any direct link to the control of cell cycle events remains to be established.