White spot disease (WSD) has been reported to cause severe mortality in farmed shrimp especially black tiger shrimp in many countries. WSSV is responsible for huge economic loss in the shrimp culture industry worldwide. The present study was carried out to examine the antiviral activity of a large scale produced plant extract of Cynodon dactylon on white spot syndrome virus (WSSV) in black tiger shrimp Penaeus monodon by in vivo testing after administration through oral route. The plant extract isolated from C. dactylon was incorporated with artificial pellet feed at a concentration of 1% or 2%. For experimental challenge shrimp were fed with WSSV-infected shrimp meat. WSSV-infection was confirmed by the PCR technique, bioassay and Western blot analysis at the end of the experiment. The results of the present study showed that the plant extract of C. dactylon was found to be highly effective in preventing WSSV infection with no mortality and no signs of WSD at 2% and 40% mortality at 1% in black tiger shrimp P. monodon, respectively. © 2008 Elsevier B.V. All rights reserved.