Sexually mature males of the freshwater prawn, Macrobrachium rosenbergii, appear in three distinct morphotypes which coexist within populations. Two of the morphotypes, the Blue-clawed and the Orange-clawed, overlap in size but differ in claw to body length ratio. The effects of size and morphotype on dominance rank were studied in groups composed of these two morphotypes, each represented by prawns in three size categories. Blue-clawed males dominated Orange-clawed males irrespective of size. The effect of size was more pronounced within morphotypes. The effect of morphotype can be interpreted as an advantage for Blue-clawed males because of their larger claws. The alpha position, occupied by the largest Blue-clawed male, was evident from the first daily observation. A linear rank order was evident only in the following 2 days. The consequences of dominance were reflected in competition for resources, where the alpha male clearly had priority of access to food and shelter.