OBJECTIVE: To evaluate the antimicrobial activity potential of the essential oil of rosemary specifically for its efficacy against the drug-resistant mutants of Mycobacterium smegmatis, Escherichia coli, and Candida albicans. METHOD: Antibacterial, antifungal, and drug resistance-modifying activity was evaluated both qualitatively and quantitatively following disc diffusion and broth dilution assay procedures. RESULTS: The rosemary essential oil was found to be more active against the gram-positive pathogenic bacteria except E. faecalis and drug-resistant mutants of E. coli, compared to gram-negative bacteria. Similarly, it was found to be more active toward nonfilamentous, filamentous, dermatophytic pathogenic fungi and drug-resistant mutants of Candida albicans. CONCLUSION: Our findings suggest that characterization and isolation of the active compound(s) from the rosemary oil may be useful in counteracting gram-positive bacterial, fungal, and drug-resistant infections.