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The influence of Mg2+-limitation on the growth of a typical Gram-positive organism—Bacillus subtilis—was investigated and the data compared with that obtained with Aerobacter aerogenes grown under similar conditions. The magnesium contents of both organisms varied with growth rate but were very similar at corresponding growth rates. With Mg2+-limited chemostat cultures of each organism, uptake of Mg2+ was almost complete at specific growth rates less than 0·7 × maximum. Cellular Mg2+ was tightly bound, none being removed by suspension of the organisms at 20° in 0·85% (w/v) NaCl. When Mg2+-limited organisms were suspended in environments containing Mg2+, this ion was rapidly adsorbed; the amount adsorbed varied with both the initial extracellular Mg2+ concentration and the composition of the diluent. B. subtilis had a greater capacity for Mg2+ adsorption than A. aerogenes but its affinity for this ion was less. The latter difference correlated with the ability of A. aerogenes to outgrow B. subtilis rapidly in Mg2+-limited chemostat cultures containing both organisms. The significance of these results is discussed in relation to the reports from other laboratories concerning differences between Gram-positive and Gram-negative bacteria in Mg2+ content, uptake of Mg2+ and ability to grow in media of low Mg2+ content.