Wireless sensor networks (WSNs) have gained much attention due to large number of applications. The WSN systems are usually deployed in hostile environments where they encountered a wide variety of malicious attacks. In order to protect the transmitted messages between any two adjacent sensor nodes, a mutual authentication and key exchange protocol is required for wireless sensor networks. Because some nature restrictions of sensor nodes which include low power, less storage space, low computation ability and short communication range, most existing protocols attempt to establish a pairwise key between any two adjacent sensor nodes by adopting a key pre-distribution approach. However, this approach has some inherent drawbacks. With rapid growth of cryptographic techniques, recent results show that Elliptic Curve Cryptography (ECC) is suitable for resource-limited WSNs. In this paper, we propose a scalable ID-based pairwise key establishment protocol that allows a sensor node can establish session keys with all one-hop adjacent nodes by one broadcast message. Compared to other existing protocols, our protocol offers two important advantages: (1) each node requires only constant memory storage, so the proposed protocol is scalable; (2) it ensures that a sensor node can directly establish secure communications with other adjacent nodes, so that the entire connectivity of the network is guaranteed. We show that it can withstand the compromise attack with captured nodes. A performance analysis demonstrates that our protocol is well suited for resource-limited WSNs.