A DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN TROPICAL ANIMAL PRODUCTION OF SOKOINE UNIVERSITY OF AGRICULTURE. MOROGORO, TANZANIA.The production of milk from cows in Tanzania has not reached the optimum levels due to several constraints including poor genetic potential of animals, lack of feeds especially good quantity and good quality during the dry seasons, prevalence of diseaseases, generally poor management and lack of good husbandry practices. Mastitis is one of the important disease that lowers the milk production and quality in local and dairy cattle in Tanzania. This cross sectional study was conducted between November 2013 and February 2014 in Kilosa and Mvomero districts in Morogoro Region; Lushoto and Handeni districts in Tanga Region. The objectives of the study were to establish the prevalence of mastitis in lactating cows, assess the types of mastitis and establish the bacterial contamination in raw milk. A total of 78 cows were examined for clinical and subclinical mastitis using udder palpation and California mastitis test respectively. Subsequently, milk samples were collected for bacteriological analysis using standard protocols. It was found that the prevalence of mastitis was 75.6% of the total samples examined; 12.8% of the examined cows had clinical mastitis while 62.8% had subclinical mastitis. Breed wise, the prevalence of mastitis was 82.1% and 72% in dairy and local cows respectively. Handeni district had the highest prevalence of mastitis (88%) while Kilosa district had the lowest prevalence (60%) within districts samples analyzed. Lushoto and Mvomero had prevalence of 70% and 83% respectively. Bacteria isolated were Staphylococcus aureus (34.6%), Streptococcus epidermidis (51.3%) and Escherichia coli (12.8%). For the first time in Tanzania, this study isolated Listeria spp. in particular Listeria monocytogene (32.1%), Listeria ivanovii (15.4%) and Listeria inoccua (30.8%) from raw cow milk. This study concludes that the magnitude of mastitis in lactating cows in the study areas was high. It is recommended that livestock experts in the districts should apply different methods to prevent and control mastitis in cattle and further studies are needed to establish the virulence and behaviour of different pathogenic microorganisms that cause mastitis in dairy cattle in Tanzania.