RESUMEN
1-If Histophilus somni can be involved ofmastitis in Holstein dairy cattle. 2- The frequency ofHistophilus somni and other bacteria isolated in mastitiscases. Cross-sectional. 116 Holstein dairy cattle. Cows were examined in 6 dairy cattle herds insuburb of Mashhad, Iran. They were tested for clinical [No:22; 18.64%] and sub clinical mastitis [No: 94; 81.04%] byusing routine clinical methods and California Mastitis Test [CMT]. They were cultured in Brain Heart Infusion Yeastagar at 37° C in 10% Co2 for Histophilus somni and otherbacteria infection. They were evaluated after 24- 48 hours. Chi- square and Fisher tests.Histophilus somni was isolated from only 2[1.73%] clinical mastitis cows. Sub clinical mastitis cows were notinfected by Histophilus somni. There were significantdifferences between clinical and sub clinical mastitis inrespected to mammary gland infection [P<0.05]. Theinfection rate in winter and spring seasons were 1.73% [No:58] and [1.73%], respectively. However, 8 bacteria wereisolated, too. The infection rate of mastitis due toHistophilus somni was very low [1.73%]. It was cultureddifficulty. The real infection rate may be higher. It isnecessary to use more sensitive methods