For the first time the occurrence of a chytrid, Batrachochytrium dendrobatidis (Bd) fungus infection in the fresh water prawn, Macrobrachium rosenbergii in grow out farms is reported. An extensive survey carried out between 2007 and 2011 in 15 farms (F) indicates that B. dendrobatidis is responsible for major pandemics in M. rosenbergii culture ponds as evidenced by the unique characteristics of the presence of fungal hyphae in claws and other features in the infected prawn. The pathogenicity of the fungus, LD50, histopathology, hematology and immunological response of the infected prawn is reported; the clinical symptoms include dull body color, abnormal protrusion associated with a grayish white color, wooly appearance with radiating grayish white fungal filaments on claw. Histopathology studies revealed that the infection resulted in faded eye color, occurrence of dark spots and melanization on the cephalothorax and muscles, translucent musculature, cuticular erosion, tissue atrophy with proliferated and infiltrated cells causing distension and gross enlargement of gill lamellae. An optimum dose of 10(5) zoospores/prawn (LD50) resulted in 50% mortality from 4 to 6 days. At high dose of 10(8) zoospores/prawn, the infection caused total mortality from 2 to 4 days. After 2nd week the total hemocyte counts (THCs) significantly decreased while the phenoloxidase (PO) level, relative superoxide anion production level, and superoxide dismutase (SOD) activity registered a significant increase (p < 0.05) when compared to control. The cumulative mortality was 90% when challenged with Bd for 30 days. The prevalence of infection was high in cooler or winter season (October, November, and December) and low in hot or summer season (March, April, May, and June) indicating a seasonal influence wielding a significant effect on mortality, hematology, and immunology profiles. Statement of relevance: In India about 1000 ha of shrimp/prawn ponds were affected by the disease, resulting in an annual loss of Rs. 10 million (US$0.23 million) during 1994-95 and Rs. 25 million (US$0.58 million) during 1995-96 (Ayyappan, 2000). Intensive prawn farming favors the rapid deterioration of environmental conditions which increase the incidence of diseases and failure of culture. Therefore augmenting the immune resistance and anti-environmental stress of prawns are of primary concern. Chytridiomycosis is an infectious disease caused by the chytrid, Batrachochytrium dendrobatidis; it is a non-hyphal zoospore fungus belonging to Chytridiomycota (Canter-Lund and Lund, 1995) or Blastocladiomycota (James et al., 2006) phylum. Chytrids are a diverse group of microscopic fungi (Barr, 1980; Barr, 2001; Kendrick, 1992) that disperse in an aqueous environment by motile zoospores with a single posterior whiplash flagellum (Sparrow, 1960). Chytrids have been well documented in amphibians (Longcore et al., 1999; Berger et al., 1999). A number of fungal pathogens were isolated and identified from prawns and shrimps (Krishnika and Ramasamy, 2014; Karthikeyan and Gopalakrishnan, 2014; Hatai, 2012; Cruz da Silva et al., 2011; Ramaiah, 2006). Recently, fungal pathogen like Aspergillus awamori has been isolated from marine shrimp, Liptopenaeus vannamei (Karthikeyan et al., 2015) and Gilbertella persicaria from Penaeus monodon. To our knowledge there is no report of the Batrachochytrium dendrobatidis infection in fish, prawn, and shrimp. (C) 2016 Elsevier B.V. All rights reserved.