Technical Abstract: Toxoplasma gondii infection in marine mammals is important because they are considered as a sentinel for contamination of seas with T. gondii oocysts, and toxoplasmosis causes mortality in these animals, particularly sea otters. Seroprevalence of T. gondii infection was determined in 75 captive marine mammals from four facilities in south and central geographical regions in Mexico using the modified agglutination test (MAT). Antibodies (MAT, 1:25 or higher) to T. gondii were found in 55 (87.3%) of 63 bottlenose dolphins (Tursiops truncatus truncatus) and three of three Tursiops truncatus gilli, two of four California sea lions (Zalophus californianus), but not in three West Indian manatees (Trichechus manatus), and two Patagonian sea lions (Otaria flavescents). Seropositive marine mammals were found in all four facilities sampled. All marine mammals were apparently healthy and there has not been any case of clinical toxoplasmosis in the facilities sampled for at least the last 15 years. This is the first report of T. gondii infection in marine mammals in Mexico.