At water temperatures below 17 degrees C, yeast infections often occurred in 6 to 11 mo old giant freshwater prawn Macrobrachium rosenbergii (8 to 12 cm) in Taiwan from May 2001 to December 2003, with a cumulative mortality of 20 to 95%. Gross signs of disease included milky hemolymph, a yellow exoskeleton, opaque muscles, and a swollen hepatopancreas (HP). Histopathology included marked edema and extensive necrotic lesions associated with large numbers of yeast aggregates and inflammation within the muscles, HP, and other internal organs such as the heart, ovary, and intestine. Yeast cell numbers isolated from various tissues ranged from 4.5 x 10(8) to 9.0 x 10(9) colony forming units (CFU) per 100 mg. From diseased prawns from 12 affected farms, the presence of Metschnikowia bicuspidata (98.4% prevalence), Saccharomyces cerevisiae (0.8% prevalence), and Candida albicans (0.8% prevalence) was confirmed by biochemical tests and sequencing of PCR products from the D1/D2 domain of 26S rDNA. Experimental infection with these isolates caused gross signs and histopathological changes similar to those observed in naturally infected prawns, and lethal doses (LD50) were 3.8 x 10(3), 2.0 x 10(3), and 4.3 x 10(3) CFU prawn-1, respectively. Although the results of this study revealed that M. bicuspidata may be the major cause of yeast infections in the giant freshwater prawns in Taiwan, this is the first time that S. cerevisiae and C. albicans are also reported as pathogens.