A long-term environmental study was conducted to evaluate the significance of water temperature, pH and salinity on the prevalence of Streptococcus iniae mortalities in barramundi sea-cage facilities. It was determined that there is a strong association between temperature and increased mortality, specifically between 25 and 28 °C. Temperatures outside of that range result in decreased mortalities attributed to S. iniae. There was no statistical significance between pH or salinity and S. iniae induced mortality (p > 0.05), although acidic conditions (< pH 6) occurring due to anthropogenic disturbances did result in acute mortalities and S. iniae was recovered from approximately 70% these fish. Laboratory challenge studies confirmed the temperature dependence of S. iniae infections as well as the increased susceptibility of barramundi to S. iniae during acid water conditions.Aquaculture Volume290,Issues 3-4,19 May 2009,Pages 224-228.Erin Bromage and leigh Owens. Microbiology and Immunology, School of Veterinary and Biomedical Sciences, James Cook University, Townsville, Queensland 4811, Australia Received 20 November 2008revised 17 February 2009accepted 18 February 2009.Available online 5 March 2009.