Background:  Staphylococcus aureus has a huge armory of virulence factors which are under the control of the quorum-sensing accessory gene regulator (agr) system. agr dysfunctional strains usually have a higher ability to form biofilm. The aim of the work was to detect the association of agr groups, agr functionality and biofilm formation among methicillin resistant and methicillin sensitive Staphylococcus aureus (MRSA/MSSA) isolated from clinical and nasal carrier specimens. Methods: Antibiotic susceptibility testing (AST) was performed to 100 clinical samples and 50 nasal carriers. Isolates were then characterized by agr typing using multiplex PCR. agr activity was evaluated using agr CAMP assay. Biofilm formation was determined phenotypically by microtiter plate method and genotypically by amplifying icaA and icaD genes. Results: A high level of resistance to different classes of antibiotics was detected. Methicillin resistant Staphylococcus aureus was more prevalent among clinical samples than nasal samples. No vancomycin resistant Staphylococcus aureus was detected. The percentage of agr dysfunctional isolates and the ability to form biofilm were higher in clinical samples than in nasal swabs, and more prevalent in MRSA than in MSSA. agr I was the most predominantallele among all isolates. The percentage of biofilm formation was higher among non-functioning agr isolates. icaD gene was the most prevalent biofilm formation gene detected. Conclusion: The formation of the biofilm in MSSA depends on ica genes, while in MRSA a biofilm can be formed in absence of both genes. The agrII allele was statistically significant associated with strong biofilm formation (p < /em> =0.001) and with agr dysfunction (p < /em> =0.030).