This article describes the laboratory findings of the short-term effects of three herbicides on the tropical seagrass Thalassodendron ciliatum. For three days T. ciliatum was exposed to Diuron, Fusillade (Forte) and 2, 4-D amine, either individually or in combination. The toxic effects were investigated by measuring the effective quantum yield through rapid light curves and the maximum quantum yield (Fv/Fm), before and after exposure at intervals of 2, 4, 6, 8, 24, 48 and 72 hrs. During the recovery phase both Fv/Fm and effective quantum yield were measured after 2, 4, 6, 8, 24, 48 and 72 hrs. Results revealed an inhibition of both ETRmax and Fv/Fm after diuron exposure. No effect on ETRmax and Fv/Fm were observed when T. ciliatum was exposed to Fusilade Forte and 2, 4-D amine. Any combination of the herbicides that involved Diuron showed inhibition both in ETRmax and Fv/Fm. Exposure to a combination of Fusilade Forte and 2,4-D amine had no effect on both ETRmax and Fv/Fm of T. ciliatum. It is concluded that, diuron is toxic to T. ciliatum at a concentration which can be found in a polluted environment while the other herbicides did not show inhibition of the parameters measured. K E Y W O RDS: Thalossodendron ciliatum, herbicides, toxicity, Fucilade, 2,4-D amine I N T R O D U C T I O N Seagrasses are among the most productive ecosystems in the shallow water marine envi-ronment. It provides habitat to a number of flora and fauna species and stabilises the sediments. Its survival is both of ecological and economic-al importance. The decline of seagrass in the past decade has mainly been attributed to anthropogenic activities. Excessive input of nutrients in the seawater has lead to an increase in epiphytic load on the seagrass that ultimately reduces light (Walker et al., 1999). Other factors in some areas may also contribute to the fast decline of seagrass. One of these can be the impact of the agricultural activities that involve utilisation of large amounts of herbicides that can leak into the seawater through runoff water or ground water seepage. For example, in Northern Queensland diuron herbicide was detected in seagrass tissue and sediments (Hynes, et al., 1998). A serious risk posed to the marine environment is the fast growing developmental activities along the coastal regions in most developing countries bordering the Western Indian Ocean. Also agricultural activities along