This document presents a performance evaluation of three MANET routing protocols (AODV, OLSR, and ZRP) under a blackhole attack, a severe security challenge characterized by a malicious node falsely claiming the shortest route to a destination. Using the Opnet simulator, the study examines how these protocols perform in terms of network load, end-to-end delay, and packet delivery ratio, revealing that AODV consistently outperforms both OLSR and ZRP in the presence of this attack. The findings suggest that AODV maintains better performance under attack, while further analysis with different protocols and attack types is recommended for future research.