Toxoplasma gondii infection in marine mammals is intriguing and indicative of contamination of the ocean environment and coastal waters with oocysts. In a previous study, 138 of 141 (97.8%) bottlenose dolphins (Tursiops truncatus) from the coasts of Florida and California had antibodies to T. gondii by the modified agglutination test (MAT). Although the MAT has been found to be highly sensitive and specific for T. gondii antibodies from several species of terrestrial animals, it has not yet been validated for T. gondii infections in marine mammals. Furthermore, T. gondii has yet not been isolated from dolphins. In the present study, sera from 146 (60 from the 2004 samples and 86 from the 2003 samples) T. truncatus from the coastal areas of South Carolina and Florida were tested for antibodies to T. gondii. Sera from 2004 were tested by the MAT, the indirect fluorescent antibody test (IFAT), the Sabin–Feldman dye test (DT), an indirect hemagglutination test (IHAT), an enzyme-linked immunosorbent assay (ELISA), and Western blot. All 60 dolphins were seropositive, with MAT titers of 1:20 in 3, 1:40 in 19, 1:80 in 29, 1:160 in 2, 1:1,280 in 3, 1:2,560 in 2, and 1:5,120 or higher in 2, and these results were confirmed in another laboratory. The DT titers of these dolphins were <1:10 in 53, 1:800 in 3, 1:1,600 in 2, and 1:3,200 in 2. The IHAT titers were <1:64 in 52, 1:128 in 1, 1:512 in 2, and 1:2,048 in 5. The IFAT titers were <1:20 in 3, 1:20 in 11, 1:40 in 36, 1:80 in 2, 1:160 in 1, and 1:320 or higher in 7. All 7 DT-positive dolphins had high MAT titers, but 2 were negative by the IHAT. Western blot results closely followed MAT results; ELISA results matched MAT results, which were 1:40 or higher. In sera from the 2003 samples, MAT antibodies were found in 86 of 86 dolphins with titers of 1:25 in 29, 1:50 in 23, 1:100 in 27, 1:200 in 3, 1:1,600 in 1, and 1:3,200 in 3; these sera were not tested by other means. Overall, MAT antibodies were found in all 146 dolphin sera tested. Because marine mammals are considered sentinel animals indicative of contamination of the coastal and marine waters by T. gondii oocysts, serologically positive infections need to be validated by the detection of T. gondii organisms in the tissues of seropositive animals.