Wireless Sensor Network (WSN) consists of a large number of small sensor nodes (SNs) that are usually deployed in hostile environments. These nodes are vulnerable to a number of security attacks and it is difficult to implement complex cryptographic schemes in SNs due to their resource constrained nature. Hence, WSNs need energy efficient and secure communication schemes in terms of storage space, power consumption and operation speed. In this work, we propose a secure and lightweight encryption scheme based on chaotic map and genetic operations. We also present the results of security and performance evaluation of our proposed encryption algorithm.