ABSTRACT. Brito D.A.P., Oliveira I.S.S., Brito D.R.B. & Costa F.N. [Prevalency and etiology of dairy cattle mastitis in the Island of São Luís in the State of Maranhão, Brazil.] Prevalência e etiologia da mastite em bovinos leiteiros da Ilha de São Luís, estado do Maranhão, Brasil. Revista Brasileira de Medicina Veterinária, 36(4):389-395, 2014. Departamento de Desenvolvimento Educacional, Instituto Federal do Maranhão, Avenida dos Curiós, s/n, Vila Esperança, São Luís, MA 65095-460, Brasil. E-mail: email@example.com The aim of this study was to investigate the prevalence and etiology of bovine mastitis in the dairy region of the island of São Luís, Maranhão State. We examined 217 cows from 14 farms located in São Luís, Paço do Lumiar, São José de Ribamar and Raposa, using a strip cup and California Mastitis Test (CMT). Milk samples were collected from positive mammary quarters and subjected to microbiological analysis. Of 857 rooms mammary evaluated, 27 (3.12%) had clinical mastitis and 215 (25.08%) subclinical mastitis. In samples of reagents to CMT, 69 showed score 1+, 103 scores 2 + and 43 score 3 +. The milk positive samples for the tests were subjected to microbiological tests for the isolation and identification of infectious agents. Of the 242 samples of milk examined, 204 (84.29%) had growth of microorganisms and 38 (15.70%) had not growth. The microorganisms isolated were coagulase negative Staphylococcus (26.42%), Staphylococcus coagulase positive (14.23%), S. aureus (14.23%), Streptococcus spp. (20.33%), Corynebacterium spp. (3.25%), yeast (2.44%) and Gram negative bacilli (1.22%). The results show a high frequency of breast quarters with clinical and subclinical mastitis and Staphylococcus coagulase negative was the etiologic agent most frequently isolated was, indicating the need for adoption of control measures aimed at improving hygiene conditions during milking.