- ABSTRACT Adult males of the fresh-water prawn Macrobrachium rosenbergii in a single-age population can be categorized into three morphotypes. Each morphotype represents a different stage in the development of the adult males from small males (SM) through orange-claw (OC) males to blueclaw (BC) males (Cohen et al., 1981; Ra'anan, 1982). All males are capable of developing through all of the above stages, but individual males largely differ in the rate in which they transform from one morphotype to another. It was observed that SM and BC males are more sexually active than OC males, while the OC males grow more rapidly than SM and BC males (Ra'anan and Sagi, 1985). To examine further this behavioral observation, two internal organs in each of the morphotypes were measured and compared. The development of the reproductive system (testes, sperm ducts, and ampullae), as an indication of sexual activity, was compared with the development of the midgut gland (hepatopancreas), an organ which is suggested to be in correlation with somatic growth. The hepatopancreas is significantly larger and the reproductive system is relatively smaller, in relation to body size, in OC males. The opposite is true of SM and BC males, in which the reproductive system is significantly larger while the hepatopancreas is much smaller in relation to body size. Thus, the relative sizes of the two organs of an individual prawn are closely associated with its position in the male developmental pathway from the SM through OC to the BC morphotype. Further, the changes in specific ratio between the two organs reflect the inverse relationship between reproduction and growth.