The e¡ect of methyl farnesoate (MF) administration on the vitellogenesis of the penaeoidean shrimp, Sicyonia ingentis, was studied. The short-and long-term treatment e¡ects as well as the e¡ect of two MF injection regimens (0.1 and 1.0 mg MF/injection) were evaluated. The studies were also carried out to understand the pattern of vitellogenesis in eyestalk ablated adult and juvenile shrimps. A combination of endpoints, haemolymph vitellogenin (Vg) levels, gonadosomatic index (GSI) and histology, was used to study the e¡ect of these treatments. The GSI increased in all the MF-treated shrimp compared with the control shrimps. Although haemolymph Vg levels declined over the experimental period in all the treatments, the Vg levels decreased signi¢cantly only in the short-term treatment with 1.0 mg MF. Similarly, haemolymph protein level also declined over the experimental period in all the treatment groups. However, except in the long-term treatment with 0.1 mg MF, all treatments showed a signi¢cant decrease in haemolymph protein level. Conversely, in all eyestalk ablated adults and juveniles, haemolymphVg, total protein and GSI increased over the experimental period, all of which were higher than the concurrent control. The discrepancy in the vitellogenic pattern between MFtreated and eyestalk ablated shrimp was possibly due to the di¡erence in the ovarian phase of the initial control. Although unilateral eyestalk ablation failed to induce vitellogenesis in juveniles, bilateral ablation induced vitellogenesis, which indicates that juveniles are competent to undergo vitellogenesis. Oocyte growth stimulation by methyl farnesoate B C Paran et al.