A study was conducted to investigate the efficacy of fosfomycin in controlling streptococcosis in Nile tilapia (Oreochromis niloticus). The minimum inhibitory concentrations (MICs) of fosfomycin against multiple S. iniae isolates showed a sensitivity range of 12.5- 25 'g/mL. The fosfomycin dose levels tested were 40, 60, and 80 mg of active ingredient per kilogram fish per day. Administration
of medicated feed started one day after infection by immersion exposure to S. iniae and continued for eight days. Survival rates of fosfomycin treated groups were 90, 100 and 96.6% with an average survival of 95. 53 %. On contrary, survival rates of infected non treated groups were 3.3%. All survivors and negative control groups showed no clinical signs, no gross pathology together with negative S. iniae re-isolation.