Toxoplasma gondii is now recognized as an important pathogen in costal marine mammals. Oocysts from cat feces are believed to be washed into seawater and serve as a source of infection via transport hosts. Experimentally, it has been demonstrated that T. gondii oocysts can sporulate in seawater and remain infectious for mice for up to 6 mo. The present study examined the long-term survival of T. gondii in seawater (15 ppt NaCl) kept at 4 C or at room temperature. Oocysts kept at 4 C for 24 mo were orally infectious for mice, while those kept at room temperature for 24 mo were not.