Abstract:DCMU-enhanced chlorophyll fluorescence ratio of marine algae [Halymenia sinensis (red alga), Ulva pertusa (green'alga), Underia pinnatifita (brown alga) and Spirulina platensis (blue-green alga)] was studied. It was found that the red alga had the highest ratio (3. 9) among these species. The green one and blue-green alga had the value of 2. 5 and 2. 8 respectively. The ratio for U. pinnatifida (collected in summer season) was as low as 1. 7. FD/FN values declined obviously under the effects of stresses (dehydration or heat). If also reduced when the algae senesced. So this ratio can be used as a sensitive index of photosyhthetic capacities of marine algae. In addition, the FD/FN values of two phytoplanktons (red tide species) Prarocentrum micans and Chaetoceras minutissmus both in batch culture, and their relation to growth rate were investigated. The resnlts indicated that both FD/FN value and U changed with the time of culture, but the important fact was that the maximum value of fluorescence ratio was reached always prior to the appearing of the maximum of growth rate. This results presented a good prospect for using fluorescence in vivo in the research of red tide occurence.