Surface water was sampled at the three red tide stations during the Guinardia delicatula bloom in Jiaozhou Bay on February 9th-10th，2004 and the colloidal substances were separated by filtration (0.7μm) and cross-flow ultrafiltration (1000 Daltons). The organic carbon in the filtrates and retentates were analyzed respectively. Heterosigma akashiwo and Skeletonema costatum were cultured in the <1000Daltons ultrafiltrate and the retentates with different amount of colloidal substances (1000 Daltons-0.7μm). The results show that the dissolved organic carbon concentration is enhanced in the water, especially the colloidal organic carbon is increased severalfold during the Guinardia delicatula bloom. Compared with the controls, the maximum growth rate μmax and maxium biomass Bf of the Heterosigma akashiwo cultured in the retentates increase markedly with the colloid amout increase, but those of the Skeletonema costatum decrease remarkably with the colloid amout increase. It indicates that the colloidal substances in Guinardia delicatula bloom water can enhance the growth of Heterosigma akashiwo but suppress the growth of Skeletonema costatum. This phenomenon implies that Guinardia delicatula is able to produce colloid size allelopathic substances during the algal proliferation. These substances could influence other algal growth, which maybe play an important role in the Guinardia delicatula bloom formation.