SUMMARY: Magnesium-limited mixed cultures, each containing two microbial species, were produced by exchange of small numbers of organisms between magnesium-limited chemostat cultures of Aerobacter aerogenes, Pseudomonas fluorescens, Bacillus subtilis, B. megaterium, Staphylococcus epidermis and Torula utilis. The fate of each species in these mixed cultures was determined following cross-inoculation. Gram-negative organisms invariably outgrew the Gram-positive ones, but the ability of B. subtilis and B. megaterium to outgrow each other, or to outgrow the yeast, depended on the inoculum size. This dependence resulted from the presence of specific extracellular products in the Bacillus cultures which stimulated their growth and uptake of magnesium. The concentration of these extracellular growth-promoting substance(s) in magnesium-limited cultures of B. subtilis varied with population density. Thus, when the magnesium concentration in the medium was decreased from 0.9 to 0.15 μg./ml. (thereby causing the steady-state population density to be decreased to one-eightieth the initial value), B. subtilis could no longer maintain itself against proportionately low concentrations of T. utilis. But addition of extracellular fluids from dense magnesium-limited B. subtilis cultures to magnesium-limited T. utilis cultures enabled small numbers of the Bacillus now to outgrow the yeast. From these two observations it is concluded that magnesium assimilation by B. subtilis is more dependent on extracellular substance(s) than is magnesium assimilation by either T. utilis or the Gram-negative organisms. Modification of the basic theory of microbial growth in a chemostat culture, to take account of product-stimulated substrate-assimilation, is suggested.