The excessive growth of some species from green algae,such as, Ulva,Chaetomorpha and Cladophora,has been reported in the formation of macroalgal blooms or greentide events in many parts of the world including Europe,North America,South America,Japan and Australia.In June 2008 the world’s largest greentide ever covering about 600 km² occurred along the coast of the Yellow Sea.Over one million tonnes of green algae were removed from the beach and coast,which caused huge economic loss to the local government.The cause,origin tracked and threatening to the marine environment of the greentide and the possibility of recurrence have greatly concerned government officials and scientists.During the period of green algal bloom,macroalgae could inhibit the growth of phytoplankton,which could modify the trophic mode of marine ecosystem.Ulva linza was a popular species of green tides that occurred along the coast of the Yellow Sea.This paper studies the allelopathic effects of the macroalgae U.linza on the microalgae I.galbana,in order to provide some theoretical proof for evaluating the effects of green algal bloom on the marine environment.Allelopathic effects of fresh tissue,dry powder,macroalgae culture medium filtrate and aqueous extracts of U.linza on I.galbanawere studied in the laboratory using coexistence culture system.Different initial inoculation concentrations(0,0.5,1.0,2.0 and 5.0 gwet/L)of U.linza fresh tissue,(0,0.2,0.4,0.6 and 1.2 gdry/L)of U.linza dry powder and(0,0.1,0.2,0.5 and 1.0 g/L)of U.linza aqueous extracts were used in coexistence culture systems.The results of the coexistence assays showed that the growth of I.galbana was strongly inhibited by fresh tissue,by dry powder and by aqueous extracts of U.linza.When the inoculation concentration of fresh tissue,dry powder and aqueous extracts was 5.0 gwet/L,1.2 gdry/L and 1.0 g/L,respectively,the cells of I.galbana could not grow normally or died completely within six to seven days.The effects of the U.linza culture medium filtrate on the I.galbana were also investigated to confirm the existence of allelochemicals.The growth of I.galbana was not significantly(P>0.05)lowered by macroalgae culture medium filtrate under initial filtrate addition for long time.On the contrary,the growth of I.galbana was strongly inhibited by U.linza culture medium filtrate under semicontinuous filtrate addition.It was speculated that continuous addition of allelochemicals was important to effectively control the growth of I.galbana. I.galbana was not inhibited when inoculated in the U.linza culture filtrate which was boiled at the high temperature and high pressure,which suggested that allelochemicals from the fresh tissue of U.linza were unstable and degradable at the high temperature.These tests prove that U.linza has allelopathic effects on I.galbana.These results not only provide insight into the cause and mechanism of green algal bloom,but a1so lead us to speculate and evaluate the effects of freefloating green macroalgae on the marine environment.