ABSTRACT: White tail disease (WTD) of the freshwater prawn Macrobrachium rosenbergii has recently been the cause of high mortalities in Thai prawn farms. The causative agents of this disease in other countries are M. rosenbergii nodavirus (MrNV) and extra small virus (XSV), which are usually detected using reverse transcriptase-polymerase chain reaction (RT-PCR) protocols. Using RT-PCR, most Thai post-larvae (PL) samples showing gross signs of WTD tested positive for MrNV but only a few were positive for XSV. In contrast, all tested brooder samples were positive for both MrNV and XSV. The possibility that brooders infected with MrNV and XSV could transmit the viruses to larvae and PL should be examined. Cloning, sequencing and comparison of deduced amino acid sequences of RT-PCR amplicons of WTD samples from Thailand with those of MrNV and XSV previously reported from the French West Indies and China revealed that the MrNV were closely related but not identical while those from XSV were identical. This is the first report of MrNV and XSV from Thailand.