Monsoon river prawn Macrobrachium malcolmsonii is an exotic freshwater prawn species recently introduced from Sri Lanka . The external appearance of the prawn is that of a spindle . The prawn consists of 20 pieces. Except telson , each piece has a pair of appendixes. Females and males have - shaped and Mshaped protuberances on the bases of f ifth walking legs. The surface of the gastric , cardiac and antennal areas of the carapace is rather coarse and there are several light brown mottles paralleled with prawn body on the both sides. During the fry ( 1- 3 cm) period, the body color of most individuals is light brown. Af ter sexually matured, the body color turns light brown, light blackish brown and light black. The tip of the rostrum slightly slants upward and extends forward, but neither going beyond the end of the antenna squama of the antenna seconda nor slanting higher than the tip of the coronal apophysis on the upper edge of the rostrum. The dental formula of rostrum is 7- 13/ 4- 6. Monsoon river prawn M. malcolmsonii belongs to the same genus as M. rosenbergii and M. nipponense. Though there are many similarit ies in their biological characteristics, there are also many differences in body color, rostrum, carapace and sternite pleurons, which can be obviously ident ified. This paper describes the external features and structures of M. malcolmsonii in detail, compares major characteristics of M. malcolmsonii and the other two species mentioned above. In addit ion, the questions are raised on the / male prawn specialization0 based on the second chelipod/ body length ratio.