Abstract: Negative effects of cyanobacteria on crustacean zooplankton Daphnia can be documented widely and severalm ainexplainary causes have been proposed (.i e. , toxicity, poor nutritional value, bad taste, and morphology ). But most of theprevious studies concentrated on the direct effects of cyanobacteria onDaphnia by add ingM icrocystis s. alone or inm ixtures( such as with Scenedesmus s.), inform at ion on the effects of culturem ed ia of M icrocystiss. on Daphnia was little. Th is paperw as to explore the harm ful effects of info-chem icals re leased by cyanobacterium M icrocystis aeruginosa during growtha nd to compare the differences in effects of different strains of M. aeruginosa on the crustacean zooplanktonDaphnia magna.Filtered cyanobacteria culturem edia of tw o different stra ins (m icrocystin-produc ing and m icrocystin- free) w ere used inthis study. The resultswere as fo llow:(1) Both of the culturem edia o f the two strains selected affected the grow th and productionofD. magna and the results showed that inhibitory effect occurred at high concentrations, wh ile stmi ulatory effectsw ere detected at low concentrat ions;(2) M icrocystin-producing strain affected the survival ofD. magna much more thanthe m icrocystin- free strain did;(3) No significant differences in grow th o fDaphnia were detected betw een the two strains,butm icrocystin-producing strain show ed stronger inhibitory effects on production than the m icrocystin-free stra in did.Therefore, it can be pred icted that, when cyanobacterial bloomsw ere dom inated bym icrocyst in-producing strainsM. aeruginosa,reduction in abundance and / or biomass of large filter- feeding cladocerans (such asDaphnia) could probab ly be explained by the high lethal effects ofm icrocystin; butwhen the phytoplankton commun ity was dom inated bym icrocystin- freestra ins, the foregoing phenomenon could not be explained by that, and then it should be attributed to other factors, such aslacking essential fatty acids, forming colony which can hamper ingestion or digestion, etc.