Biological early warning systems (BEWS) which detects toxicity by tracking the physiologic responses of the whole organisms has been developed in recent years. In the paper, we firstly apply the C4.5 decision tree Algorithm to the biological early warning systems and propose a new BEWS which can detect toxicity by classifying the flea's behavior data. The new system is meaningful to both biological field and data mining. The new classification application includes decision tree algorithm and feature selection. The new BEWS system consists of training and test process. The experiment results show that the proposed new bio-monitoring system is available for environmental toxicity detection.