Abstract: In order to understand the environmental conditions of the bloom occurrence of Heterosigma akashiwo growth responses of H. akashiwo to various dissoved organic nitrogen (DON) sources were assayed in laboratory experiments. H. akashiwo could utilize several kinds of DON, rank order of maximum cell number was: threonine urea alanine glutamic acid NO3-glycine. The maximum cell number and the maximum specific growth rate (max) in the medium of threonine, urea, alanine, and glutamic acid were superior to those in NO3-medium, with the relative maximum cell number of 125%~253% compared to NO3- medium. However, H. akashiwo could not maitain effective growth in serine and uric acid medium. H. akashiwo could not endure N-free or P-free or both nutrient free conditions. The results suggested that the high capasity of H. akashiwo to utilize DON made it predominance in phytoplanton competition, and eutrophication and orginc pollution in coastal areas provide basic nutrient demands for its bloom occurrence.