The effect of eyestalk ablation on morphogenesis and levels of methyl farnesoate (MF) in the hemolymph were investigated in juvenile females of the spider crabLibinia emarginata.The ablated crabs had significantly increased levels of MF in the hemolymph as early as 7 days post-ablation, which continued to increase during premolt. In the intact controls, MF levels in the hemolymph remained low throughout the intermolt and premolt periods. All ablated animals molted to giant juveniles, while the intact controls molted to the adult form. These results strongly suggest that there is a period of the molt cycle which is sensitive to elevated levels of MF in the hemolymph, and that MF has a juvenilizing effect on morphogenesis inL. emarginataand probably in other crustaceans as well.