ABSTRACT This study aimed to investigate the prevalence rate of brucellosis, leptospirosis, and chlamydiosis in the offspring of free-living sea lions (Zalophus californianus) in the Mexican Gulf of California. The work was carried out across six islands where reproduction takes place. Sixty-one blood samples were collected from seemingly healthy sea lions from six to eight weeks old. For diagnosis, serum was obtained by venae cavae puncture. In the search for smooth Brucella spp., based on phenotypic characterization, antigens were detected employing an antigen test for Brucella abortus, strain 1119-3. In the case of leptospirosis, the serological diagnosis was carried out by microscopic agglutination to identify 12 Leptospira interrogans serotypes. Lastly, Chlamydia abortus was identified using commercial ELISA (ID Screen®). All 61 serum samples presented negative results for the B. abortus antigen and the C. abortus ELISA. The serological diagnosis for leptospirosis showed that 29 out of 61 sera were negative (47.5%) for all serotypes, and 32 out of 61 were positive (52.5%) for at least one serotype. After studying the three diseases, results suggest that the presence of antigens against Leptospira is a potential threat to the protected sea lions from the Gulf of California.