Bacteria can adhere to mineral surfaces and affect subsequent flotation of the minerals. In the present study, the adhesion of Bacillus subtilis and Mycobacterium phlei onto dolomite and apatite was studied by sorption measurements and scanning electron microscopy. The effect of the microorganisms on the oleate flotation of the minerals was then compared at several different pH values. It was found that both B, subtilis and M. phlei adhered to dolomite more readily than onto apatite at acidic and near neutral pH values. At more basic pH values B. subtilis adheres more readily onto and remains a better depressant for dolomite than for apatite. However, at basic pH values, M. phlei adsorbs more onto apatite than onto dolomite with the result that it is a weaker depressant for dolomite, but a stronger depressant for apatite than B, subtilis. The differences in adsorption characteristics are attributed to the different surface characteristics of the two bacteria species and of the two minerals. Both possess acidic isoelectric points. However, B. subtilis has a greater affinity for Mg(II) ions than does M. phlei, Thus, B. subtilis should adsorb more strongly onto dolomite through Mg sites than M. phlei. M. phlei, however, has a more hydrophobic surface. The result is that both species adsorb onto dolomite and function as dolomite depressants and also function as apatite depressants, albeit as weaker depressants. The net results show that, while both function as depressants in anionic collector flotation of dolomitic phosphate ores. B. subtilis functions as the stronger depressant, especially for dolomite. Implications in the anionic flotation of apatite from dolomite are discussed.