Myrothecium roridum isolates were evaluated for their effectiveness in controlling the aggressive water hyacinth and molecular identification was conducted using internal transcribed spacer (ITS) rDNA region and amplified fragment length polymorphism (AFLP) markers. M. roridum isolates were collected from nine provinces of Thailand. Seventy isolates were included in the primary screening, using colonial growth rate and spore number measurement. The results indicate that five isolates were the most aggressive and so were selected to evaluate their effectiveness in controlling the water hyacinth. The result showed that M. roridum isolate KKFC 408 had significantly highest disease severity (P