Stimulation of ovarian maturation in the red swamp crayfish Procambarus clarkii was accomplished in three different trials by administration of methylfarnesoate (MF). After 30 days of treatment, the ovaries of prereproductive females were 2- to 10-fold larger and in later stages of vitellogenesis than those of the controls. Similar and statistically significant results were observed in a second 30-day trial, which was begun during the middle of the vitellogenic cycle. In this experiment the ovaries of the controls and the treated groups were in late vitellogenesis, and the oocyte diameters of both groups were similar, suggesting that MF stimulated more oocytes to mature. In a third experiment of 60 days duration, the ovary was again enlarged in the treated animals. In addition, MF levels in the hemolymph of treated females undergoing vitellogenesis were found to be twice as great as those of the controls. We conclude that exogenous MF can stimulate and enhance ovarian maturation. These experiments strongly support the concept that MF acts as a gondatropin in crustaceans.