Abstract: Predation of Poterioochrom onas sp. on preyMicrocystis is norm ally a one way process,.i e.,prey cells wereontinuous ly ingested and gradually disappeared. H owever,it w as not alw ays the sam e. Occasionally,a patch of bluegreenolored cells may re-appear from the bottom of the cu lture flask and gradually proliferated. Based on th is phenomenon,xpermi entswere des igned by using three strains ofMicrocy stis-M. aerug inosa AC,DS and PCC 78-0-as prey organism co-cultured with predator Poterioochrom onas sp.,respect ively. A fter one or twom onths,eventually/transformed0icrocystis populationsw ere obtained separately from each of co-cultured treatment. Nest PCR and 16S rDNA analys isdent ified these transformed cells to beMicrocystis sp.. Furthermore,it was astonish ingly found that these three transformedMcrocystis populations had lost the microcystin-producing ab ility. The result strong ly suggested that P oterioochromonasp. played an mi portant role in transform ing the toxicity ofMicrocy stis during predating process. ransformed,non-toxicMicrocystis had higher value of carote inoids/chlorophyll ratio but lower value of cyanophyc in/hlorophy ll ratio com pared with orig inal toxic strains. L ight response curves indicated that the PS?quantum yield andhotosynthetic activity of these non- toxic stra ins were higher than that of original toxic stra ins,moreover,non-toxic strainsch ieved them axmi um photosynthetic act iv ity at low er light intens ity than that of toxic strains. Th ism ight explain why theon-toxic transformed strains outgrew the toxic wild strains in Microcystis and Poteroochrom onas co-culture.