Abstract: The study was carried out to determine the effect of seasons on bacterial mastitis in dairy cows. Three years health data from A and M Dairy Farm was analyzed to provide baseline information on the incidence and seasonal pattern of mastitis. Mean annual incidence of clinical mastitis at A and M dairy Farm was 16.5%. A clear pattern of higher incidence of mastitis was observed when the ambient temperature was less than 21°C in cold months during the whole study period (3 years) at A and M dairy Farm. In another study, milk samples of new case of mastitis from A and M dairy were examined for the presence of bacteria. The annual incidence of mastitis at the J. B. Dairy Farm was 14.4%. The annual pattern of incidence of mastitis in relation to ambient temperature was similar to that observed at A and M dairy Farm. Environmental and contagious mastitis pathogens were identified in 43.3% of the clinical mastitis cases and the coliform mastitis accounted for 21.1% of the clinical cases at J.B. Dairy Farm. However, out of 90 new cases of mastitis considered at the J.B. Dairy, 21.1% were infected with coliform bacteria, 12.2% with S. haemolyticus, 11.1% due to C. bovis, 10.0% with S. aqalactiae and 45.5% with other isolates. In conclusion, the coliform bacteria were the main cause of bacterial mastitis with higher incidence during the cold months.