- The use of Rosmarinus officinalis as a treatment against Streptococcus iniae in Oreochromis sp.(tilapia) is described. Sixteen accessions of R. officinalis were extracted in several solvents and tested in vitro against S. iniae by disk diffusion assay. All extracts were effective in inhibiting bacterial growth. The most effective was accession no. 1 when extracted in ethyl acetate, which inhibited bacterial growth by 37.5 mm/mg extract. R. officinalis was found to have a bacteriostatic effect in a broth dilution assay, as the inhibited bacterial growth was recovered after 48 h. For oral administration, R. officinalis was mixed in the feed either as whole dry leaves or as dried ethyl acetate extract in a ratio of 1: 17 or 1: 24 (w/w), respectively. There was a significant (p< 0.012 and p< 0.003, respectively) reduction in mortality rate in fish fed on both diets following infection with S. iniae.