In the fall of 1987, several areas of Florida Bay were severely affected by the sudden die-off of the seagrass Thalassia testudinum Banks ex Konig (turtle grass). Although the cause is still unknown, several factors were suggested as influencing the on-set of the die-off event including increased salinity, light stress due to self-shading, and disease. Blades of seagrass plants found in the area of die-off were infected by Labyrinthula sp, a pathogenic protist. A similar die-off occurred in another species of seagrass, Zostera marina, in the 1930s that was attributed to the pathogenic protist, Labyrinthula zosterae. Zostera marina produces inhibitory phenolic acids in response to infection by L. zosterae, a response that is diminished in plants exposed to low light and high temperature. This study examined the differences in phenolic content of healthy and infected T. testudinum leaf blades in laboratory cultures to determine if T. testudinum produces a chemical defense against pathogens similar to that of Z. marina. The possible increased susceptibility of turtle grass to Labyrinthula sp. infection under high salinity and low light was also examined. In culture, infection by Labyrinthula sp. induced a rapid, short-term production of total phenolics in Thalassia testudinum under normal, non-stressed conditions. The initial induction was followed by a sharp decline. The production of individual phenolic acids was not induced by infection. In contrast, the production of caffeic acid was inhibited by infection. Environmental stress (low salinity and low light) caused a decrease in both total phenolics and several phenolic acids. Levels of PHBA, vanillic acid, and caffeic acid decreased in low salinity (25ppt) treatments, and caffeic acid decreased in response to low light stress. There was an interaction between stress and infection that resulted in higher levels of phenolics in plants exposed to infection and stress compared to those exposed to stress alone. In culture, plants did not survive exposure to high salinity (45ppt) similar to that found in Florida Bay during the die-off event