In Staphylococcus aureus, using the imidazoles miconazole and ketoconazole, detailed studies of minimal inhibitory concentrations, kinetics of growth, viability, and release of intracellular K+ confirm that the two imidazoles work differently in this bacterium. Miconazole is bactericidal at low concentrations and causes release of cellular K+. Ketoconazole has no bactericidal effect at any tested concentration and has little effect on K+ permeability of S. aureus; it slows growth at high concentration. This is reflected in a low minimal inhibitory concentration for miconazole and a high one for ketoconazole. The probable mechanisms of the bacteriostatic and bactericidal effects of the imidazoles are discussed in light of these results and the previously described antifungal mechanisms of the drugs. alpha-Tocopherol blocks the action of both imidazoles.