The phytoplankton population and organic-detritus of high-output (11.25-15 metric tons/ha. yr) fishponds with black carp as the major cultured species were studied during 1983-1984. The results indicated that the standing biomass of phytoplankton(wet) was 20-60mg/L and the standing crop of organic-detritus was 30-70mg/L. The dominant species of the studied phytoplankton community are Cryptomonas spp., Gymmodinum cyaneum. Euglena spp., Synedra spp., Nitzschia spp., Raphidiopsis spp., Merismopedia spp. and Chlorellaceae spp., which formed the water bloom alienately. The paper also studied the relationship between phytoplankton and water quality, and the relation between a bundance of phytoplankton organic-detrutus and fish output.