Technical Abstract: Streptococcus iniae is a significant bacterial pathogen that causes hemorrhagic septicemia and meningoencephalitis in tilapia, hybrid striped bass, rainbow trout, olive flounder, yellowtail, barramundi and other species of cultured and wild fish worldwide. In tilapia production, vaccination of fry against the bacteria is essential to reduce economic losses. The goal of vaccination is to elicit a strong immune responce to the administered antigen that is capable of prividing long-term protection against infection. However, information on the efficacy of modifiied S. iniae vaccines in newly hatched tilapia fry is limited.