This work is , in addition to the p hytochemical study , to evaluate the anti - staphylococcal activity of the essential oil of Rosmarinus officinalis L. and detect some kind of synergy interactions with c iprofloxacin. Essential oil is extracted by steam distillation in a Clevenger type apparat us (Clevenger, 1928), analyzed by gas chromatographic coupled with mass spectrometry. Aromatogram , minimum inhibitory concentrations (MIC ) and bactericidal (MBC ) allowing to evaluate the anti - Staphylococcus aureus activity of essential oil alone or in com bi nation with ciprofloxacin were studied . The report MBC/ MIC of the essential oil of rosemary is 1, which allows the oil to qualify as a bactericide . Ciprofloxacin is bactericidal with a report MB C/ MIC equal to 2, which confirms the summary of the charact eristics of this product. The MICs of the two products are used to calculate the fractional inh ibitory concentration (FIC) , it is less than 1 , which is used to identify a synergistic interaction between ciprofloxacin and the essential oil of rosemary . Int eraction between the essential oil of Rosmarinus officinalis L. and ciprofloxacin is synergistic. It may enhance the anti - S taphylococcal activity and reduce the number and severity of side effects of this fluoroquinolone ; especially those that are dose - dep endent.