This study was carried out on 36 dairy friesian cows suffered from astitis at Assiut Governorate. E.coli, Streptococcal uberis, Kelbsiella pneumoniae and C.bovis formed the main bacterial causes of mastitis. They were isolated and identified from 41.6%, 22.2, 22.2% and 13.9% of the total infections respectively. E.coli (54.5%) and Streptococcal uberis (27.2% were the most prevalent pathogens during the first month of lactation, while kellbsiella pneumoniae (62.5%), and Corybnebacterium bovis (66.6%) were the predominant causes from the period of 31-90 days and more than 90 days respectively. The largest percentage of intramammary infection (IMI) were
detected during the summer season (83.3%), and early stage of lactation (61.1%). Antibiogram study revealed that gentamycin, and kanamycin had highly inhibitory effect on isolated strains of E.coli, Streptococcal uberis, Kelbsiella and pneumoniae C.bovis.