The present work aimed to study the biological effects of Extremely Low Frequency Electromagnetic Field (ELF-EMF) on the growth rate, morphology and antibiotic sensitivity patterns of the Gram-positive bacterium Staphylococcus aureus to determine any morphological and metabolic changes that might have been caused by ELF-EMF. In order to compare cell viability, number of colony-forming units (CFU) and growth rate (optical density at 600nm) was determined. The results showed that a highly significant inhibition effect occurred when S.aureus was exposed to resonance of 0.8 Hz QAMW for a single exposure (120min). Moreover, exposed cells became more sensitive to the tested antibiotics compared to control. Significant ultra structural changes occurred as observed by Transmission Electron Microscope (TEM). Random Amplified Polymorphic DNA-Polymerase Chain Reaction (RAPD-PCR) revealed genetic fingerprinting variation as observed in the electrophoresis patterns between exposed and unexposed cells of S.aureus.