- Methyl famesoate (MF), the unepoxidated form of insect juvenile hormone III, was detected in larvae of the freshwater prawn Macrobrachium rosenbergii, which metamorphose to post-larvae following 11 larval stages. The possible role of MF as a morphogen was studied by administering the compound to M. rosenbergii larvae via an Artemia vector. Higher MF levels caused earlier retardation of late larval growth, and the highest dose retarded larval development. Furthermore, MF sig- nificantly affected the patterns of metamorphosis and the appearance of intermediate individuals exhibiting both larval and post-larval morphology and behavior. Three intermediate types were defined, two of which were found only at the MF-treated groups and one that was exclusive to the higher dose treatments. The relative abundance of intermediate specimens increased from 2% in the control to 32% in the high MF concentration, which suggests that MF has a juvenoid-like effect in this decapod crustacean.