Intrusion detection is a major research problem in network security. Intrusion Detection Systems (IDS), analyses information from the network trying to identify suspicious behaviors and detect intentions to attack the system. Intrusion attempts are nonlinear with an unpredictable behavior on the network traffic. The process of selecting the key features that allows discriminate attacks from normal traffic, is a crucial task in information security to obtain an effective IDS. We propose in this paper to use an Evolutionary Algorithm and an evaluation function from a classifier, to automatically select key features from a data set before defining a Decision Tree that can be used to discriminate among the network data type. The purpose of this study, is to propose an intrusion detection technique that selects key features using a specially designed evolutionary algorithm with individual evaluations done using C4.5, a wellknown classifier that discriminate data using decision trees. We report very encouraging results of our approach using NSL-KDD intrusion detection benchmark data sets.