Abstract
The allelopathic effects of the extracts of a brown alga Sargassum horneri on red tide microalgae were examined using Skeletonema costatum, Chattonella antiqua and Heterosigma akashiwo. The growth of S. costatum and C. antiqua were inhibited remarkably by the extracts of S. horneri while H. akashiwo needed higher amount of the extracts than S. costatum and C. antiqua to attain same level of growth inhibition, confirming the difference in the allelopathic effects with microalga species. The microscopic observation indicated that the growth inhibition effects of the potential allelochemicals produced by S. horneri were associated with the change of the body form of the red tide microalgae tested.