With the explosive growth in the adoption of wireless networks in the recent years, the limitations of wireless security have become a topic of wide spread concern. It is easy for hackers to enter or leave a wireless network. They can directly attack the network and inject spurious packets, tamper with packets, drop packets, or impersonate another node. This violates the network's goals of availability, integrity and authentication. This paper presents an authentication scheme for Mobile Ad hoc Networks (MANETs) that is designed to combat such attacks from adversaries. In this scheme, every packet is authenticated at every node. The performance of the proposed scheme is compared with other published schemes and it is found to be just as efficient while improving on security.