The prevalence of bovine mastitis in 284 milking cows of different local (Horro and Boran) and exotic (Jersey, Friesian, and Simmental) breeds from three livestock farms that are found in the central region of Ethiopia was examined in 1989 and 1990 in the dry and long rainy seasons using bacteriological diagnostic methods. Under infection was one of the most frequently recorded clinical cases in the survey areas. A detailed study of the disease was required to formulate possible control measures. An average of 5.3 percent cows and 1.9 percent quarters were clinically infected and 19 percent cows and 7.4 percent quarters subclinically infected. The local breeds were found to be less susceptible to mastitis than the crosses. Animals of the Friesian cross-breed had the highest mastitis incidence followed by the Simmental cross-breed. The following genera were isolated as causative agents of the disease: Staphylococci, Streptococci, Corynebactera, Pasteurella, and Nocardia. Prevalence of infection was related to management.