Abstract: Allelopathic effects of dry powder and extract of acetone/ethyl acetate of Eichhornia crassipes roots on Alexandrium tamarense(HKstrain)were evaluated 0.1 g,0.5g,1.0g,1.5g and 2.0g/L of E1crassipes root powder were added,respectively1Due to low solubilityof extracts,a control of slide was used as carrier to measure the actual concentration of extracts in the cultures.At first,0.01g,0.03g,0.05g,0.07g/L of extractswere put into the weighted slides1Then the slideswere added to the flasks containing 100mL of A1tamarense1At the endof the experiment,the slideswere takenout,dried andweighted1The actual concentrationof extracts in the cultures wasobtainedfrom the loss in weight1The results showed that the growthof A1tamarense was absolutely inhibited when the algae was exposured to dry powder of E1crassipes roots at above 1.5g/L1When the concentration of dry powder of E1crassipes roots were 0.5g/L and 1.0g/L,the growth of algae increased after 3 days1The growth of A1tamarense was also inhibited at the actual concentration of 0.038g/L and 0.022g/L extract of acetone and ethyl acetate of E1crassipes roots,respectively. It suggested that E cras-sipes could be a potential algaecide to control harmful algal blooms