A conventional method for evaluating chemical toxicity on algae is the 72 h algal growth inhibition test. However, simpler, faster, and lower-cost methods are required to evaluate the toxicity of a large number of chemicals. One simple approach is to measure algal growth with an optical technique in a small-scale algal culture. Here we propose measuring delayed fluorescence(DF) ,a kind of prolonged ultra-weak fluorescence. DF showed correlation with the number of algal cells, and the detection

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... ensitivity limit was 1 × 10 4 cells/mL in our experimental setup. DF also showed unique characteristics when measuring a mixture of living and inactivated algae, and the DF signal showed a correlation with the living cell content. We also compared dose-response curves obtained from DF and cell density measurements for two chemicals(p-chlorophenol and m-chloroaniline) in a 24 h exposure test. DF measurement was more sensitive than the cell density meas urement, especially at shorter exposure times of less than 8 h. The results suggest that DF is a suitable choice of detection method that is expected to allow algal growth inhibition tests to be conducted in a shorter time.