Isolation and characterization of genes specifically expressed in ovaries are necessary for understanding sex differentiation and ovarian development processes in the giant tiger shrimp, Penaeus monodon. In this study, a transcript that significantly matched the polehole precursor was further characterized by RACE-PCR. The sequence obtained was 5151 bp in length and contained a coding region of 5031 bp corresponding to 1677 amino acids. This transcript was only expressed in ovaries but not in testes of juveniles (N=10) and broodstock (N=22) of P. monodon. A tissue distribution analysis further confirmed ovary-specific expression of this transcript (called P. monodon ovary-specific transcript 1, Pm-OST1) in female broodstock. Expression levels of Pm-OST 1 in ovaries of juvenile P. monodon upon 5-HT injection (33.9±6.40 g; 50 µg/g body weight) were significantly higher at 12–72 hours post injection (P<0.05). Quantitative real-time PCR indicated that Pm-OST1 was comparably expressed throughout ovarian development in normal P. monodon broodstock (P>0.05). However, the expression level of Pm-OST1 was significantly higher in stage-III ovaries in eyestalk-ablated broodstock (P<0.05). Pm-OST1 was clearly localized in the ooplasm of previtellogenic and vitellogenic oocytes. Our results suggest that Pm-OST1 plays a functionally important role in promoting the development of female germ cells and oocytes in P. monodon.