Technical Abstract: Abstract The Gram-positive coccus, Streptococcus iniae, is among the most prominent causes of septicaemic disease affecting both captive and wild fish in fresh, estuarine, and marine waters. This coccus is highly virulent for more than 27 species of fish, worldwide. Resistance to antibiotics, refusal of sick fish to eat mediated (medicated?) feed, and the rapid onset of mortality make the inadequate antibiotic therapies. Vaccination is the best means of preventing streptococcosis. Vaccines against S. iniae have been successfully developed and employed in fish production systems. The vaccines consist of either formalin-killed S. iniae cells alone (bacterin) or combined with extra-cellular components (modified bacterin). Mixing The addition of an adjuvant into the bacterins may extent their effectiveness. Bacterins are administrated by injection and provide protective immunity between 4 and 6 months. Specific serum antibodies against S. iniae is produced following the administration of the and appears to be responsible for the protective immune responses. This antibody alone or aided by complement may kill the streptococcal cells either in the presence of absence of phagocytic or natural cytotoxic cells. The well being of fish calls for prevention of infection by vaccination, quarantine of new arrivals, health and water quality monitoring, management of husbandry stress, and a routinist disinfection of facilities and equipment on an as used basis. Adoption of such a health management plan is a much more cost-effective alternative to depopulating and fallowing a highly infected production facility.