Harmful algal blooms resulting in red discoloration of coastal waters in the Persian Gulf, Iran were first observed in January 2007. The species responsible for the bloom, which was identified as Cochlodinium polykrikoides, coincided with massive aquatic organisms’ mortalities in the Persian Gulf. The effects of water soluble extract (0.2, 0.6, 0.8 and 1.6 g L-1) from 6 species of marine macroalgae; Ulva lactuca, Enthromorpha intistialis, Colpomenia sinuosa, Sargassum illicifolium, Gracilaria corticat and Hypnea valentiea , fresh thallus (2.5 and 5.0 g L-1) and macroalgal culture medium filtrate from 3species of marine macroalgae; E.intistialis, C.sinuosa, and H.valentiea on growth of Cochlodinium polkrikoides (Dinoflagellate) were investigated for 15 days in co-culture under controlled laboratory conditions. The results clearly showed that the growth of C. polkrikoides was significantly inhibited by the water-soluble extracts of seaweed at relatively all concentrations in contrast to control with any seaweeds extract (p<0.05). The growth inhibition of C. polykrikoides was significantly higher in co-culture with fresh thallus of E.intistialis, C.sinuosa and H.valentiea (p<0.05). In macroalgal culture medium filtrate the highest growth inhibition of C. polykrikoides was obtained in co-culture with E.intistialis and was significantly higher than the other macroalgal culture medium filtrate of seaweeds(p<0.05). Therefore, we could conclude that using the tested seaweeds either as an extract or in co-culture with fresh thallus could be used as an alternative to biological control of C. polykrikoides
