In May 2001, an epizootic yeast and bacterial co-infection in the giant freshwater prawn Macrobrachium rosenbergii occurred in Taiwan causing a cumulative mortality of 25%. The diseased prawns had a yellowish-brown body color, milky hemolymph, opaque, whitish muscles, and were approximately 7 mo old with total lengths ranging from 8 to 10 cm. Histopathological examination showed marked edema, yeast infiltration, and necrotic lesions with inflammation in the muscles, hepatopancreas and other internal organs. We isolated 2 pathogens from the diseased prawns, one was a yeast (AOD081MB) and the other a gram-positive coccus (AOD081EF). The gram-positive coccus was identified as Enterococcus faecium by the API 20 Strepsystem, conventional biochemical tests, and it had 99% 16S rDNA sequence identity (GenBank Accession Number AJ276355) to E. faecium (GenBank Accession Number AF529204). The sequence of a PCR product from the D1/D2 domain of 26S rDNA (GenBank Accession Number AF529297) from the yeast gave 99% sequence identity to Metschnikowia bicuspidata (GenBank Accession Number U44822). Experimental infections with these isolates produced gross signs and histopathological changes similar to those observed in the naturally infected prawns. The lethal doses (LD50) for isolate E. faecium AOD081EF, M. bicuspidata AOD081MB and the co-infection were 4.7 x 10(4), 2.6 x 10(2), and 2.4 x 10(2) colony-forming units prawn(-1), respectively. This is the first report of a confirmed co-infection of M. bicuspidata and E. faecium in prawn aquaculture.