A broadcast authentication mechanism is important in wireless sensor networks, assuring receivers of a packet's validity. To provide authentication, some researchers utilize one way key chains and delayed disclosure of keys; however, such an approach requires time synchronization and delayed authentication. Another technique uses one-time signature schemes. Unfortunately, such schemes suffer from large key sizes and limited number of uses per key. To cope with these problems, we propose an efficient, one-time signature-based broadcast authentication scheme for wireless sensor networks that reduces storage usage and includes a re-keying mechanism. Keywords: Authentication, re-keying mechanism, hash trees.