Technical Abstract: Tilapia aquaculture is one of the fastest growing segments of fish production in Brazil. Nile tilapia (Oreochromis niloticus) is largely cultivated in the state of Parana, where Streptococcus agalactiae is the cause of severe disease outbreaks. The objective of this paper was to evaluate an inactivated S. agalactiae vaccine in tilapia for the control of streptococcal disease outbreaks. Tilapia, weighing approximately 20 g each, were intraperitoneally (i.p.) inoculated with 0.1 mL of the vaccine at a dose of 2.0 X 10 (8) colony forming unit (CFU) mL. One group of tilapia (treatment 1) received one vaccine dose and the other group of tilapia (treatment 2) received two doses, with an interval of 21 days. The control group was i.p. inoculated with 0.1 mL tryptic soy broth fish. Immunized and control tilapia were i.p. challenged with 0.1 mL of 3.0 X 10 (7) CFU mL at 30 days post vaccination. The fish were monitored daily for disease signs and for mortality for 16 days post challenge. A statistically significant difference (P = 0.0045) was found between the mortality of treatments 1 and 2. The value of relative per cent of survival of 83.6% and 96.4%, respectively, indicate that this vaccine was efficient in Nile tilapia.