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The quantitative relationship between the median effective concentration (EC 50 ) of organic chemicals to Daphnia magna and the number of molecular fragments was investigated based on experimental EC 50 values for 217 chemicals derived from the literature. A fragment constant model was developed based on a multivariate linear regression between the number of fragments and the logarithmically transformed reciprocal values of EC 50 . Functional correction factors were introduced into the model. The model was verified using an independent set of randomly selected data. The mean residual of the final model was 0.4 log-units. The robustness of the model was discussed based on the results of three jackknife tests.