Staphylococcus aureus is generally regarded as a leading cause of mastitis in dairy cattle. The aim of this study was to investigate the pattern of agr groups and any possible relationship between agr groups and antibiotic resistance among S. aureus strains isolated from bovine mastitis in Northeast of Iran. For this purpose, a total of 300 bovine mastitic milk samples were taken from dairy industry farms of Khorasan Razavi Province, Iran. S. aureus were isolated and identified according to the standard methods. Antibiotic susceptibility testing was conducted by disk diffusion method. In this study a total of 31 isolates of S. aureus were evaluated for agrD gene polymorphism by specific primers. Most of the isolates belonged to agr group I (54.8%), followed by agr group III (25.8%) and agr group II (19.4%). There was not any isolates belonging to group IV. Resistance to methicillin in agr group I isolates was more than other groups. Agr groups II and III were quite susceptible to methicillin. Due to high prevalent of S. aureus isolates and high antibiotic resistance rate in bovine mastitic isolates, it is important to verify the characteristics of S. aureus strains in Iran

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