The paper describes an immunogenetic approach which can detect a wide variety of intrusive activities on networked computers. In particular, this technique is inspired by the negative selection mechanism of the immune system that can detect foreign patterns in the complement (non-self) space. The novel pattern detectors (in the complement space) are evolved using a genetic search, which could differentiate varying degrees of abnormality in network traffic. The paper demonstrates the usefulness of such a technique in intrusion/anomaly detection. A number of experiments are performed using intrusion detection data sets (DARPA IDS evaluation program) and tested for validation. Some results are reported along with their analysis and concluding remarks.