In this paper, we present a novel lightweight protocol for data integrity in wireless sensor networks. Our protocol is based on a leapfrog strategy in which each cluster head verifies if its previous node has preserved the integrity of the packet using the secret key it shares with two hop uptree nodes. The proposed protocol is simple. The analysis and simulation results show that the protocol needs very few header bits, as low as three bits, thus resulting in negligible bandwidth overhead; the protocol poses very low computational overhead, it needs to compute just a hash as compared to multiple complex operations required by any cryptographic implementation for verifying authenticity.