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 (Mr NV) 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 Mr NV but only a few were positive for XSV. In contrast, all tested brooder samples were positive for both Mr NV and XSV. The possibility that brooders infected with Mr NV 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 Mr NV and XSV previously reported from the French West Indies and China revealed that the Mr NV were closely related but not identical while those from XSV were identical. This is the first report of Mr NV and XSV from Thailand. KEY WORDS: White tail disease · Macrobrachium rosenbergii nodavirus · Extra small virus · RT-PCR detection · Brooder Resale or republication not permitted without written consent of the publisherDis Aquat Org 69: [255][256][257][258] 2006 Recently, XSV and Mr NV have been purified .Detection methods for Mr NV include a double antibody sandwich enzyme-linked immunosorbent assay (DS-ELISA) (Romestand & Bonami 2003) and viral genome-based detection methods such as dot blot hybridization, in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR) amplification (Sri Widada et al. 2003). Similar genome-based detection methods are also available for XSV (Sri Widada et al. 2003. More recently a single-tube, duplex RT-PCR method has been developed for simultaneous detection of Mr NV and XSV (Yoganandhan et al. 2005).In the present study, farmed Macrobrachium rosenbergii showing gross signs of WTD and grossly normal brooders were tested for the presence of Mr NV and XSV by RT-PCR (Sahul Hameed et al. 2004a), and selected amplicons were sequenced and compared to those previously reported for Mr NV and XSV from other countries. MATERIALS AND METHODS PL and brooders.Infected PL with prominent signs of whitish muscle in the abdominal region were collected from different locations in Thailand (Table 1). In addition, 3 samples of grossly normal, pond-reared brooders were collected from culture ponds in Rachaburi, Thailand. These samples were transported to the laboratory on dry ice and stored at -20°C.Total RNA extraction. Whole PL (150 mg), hemolymph (150 µl) or tissue fragments (150 mg) from abdominal muscle tissue, tail muscle or pleopods were homogenized in 300 µl of TN buffer (20 mM Tris-HCl, 0.4 M NaCl, pH 7.4). The homogenate was centrifuged at 12 000 × g for 15 min at room temperature (27 to 30°C). The supernatant (150 µl) was extracted using 1 ml of TRIzol reagent (GIBCO-BRL) according to the manufacturer's protocol. RNA was precipitated...