Cyclin B is a well known regulatory factor that plays a crucial role in mitosis and meiosis. Although the existence of cyclin B has beenreported to be universal in a wide variety of eukaryotic organisms, no molecular data are available on crustacean species. In this study, threeforms of cyclin B transcripts were first identified and characterized in the ovary of the commercially important kuruma prawn Marsupenaeusjaponicus. The three transcripts (2.4, 1.9 and 1.7 kb) shared the identical sequence, with variations only in the length of 3V untranslatedregions (UTRs), and coexisted in the ovary as demonstrated by Northern blot analysis. The sequences of 3V UTRs indicated that the distinctlength UTRs of the transcripts is attributed to an alternative usage of various polyadenylation signals in the 3V UTR. The open reading frameof 1203 bp encoded a putative 401 amino acid peptide. The deduced amino acid sequence shared 45–50% identities with the known B-typecyclin in other animals. Quantitative real-time RT-PCR revealed that the short transcript (1.7 kb) was the most abundant among the threetranscripts, followed by the long (2.4 kb) and medium (1.9 kb), and the three forms of the transcripts displayed various expression profilesduring oogenesis. In situ hybridization showed that the short transcript commenced expressing in the ova as early as the oogonia stage andaccumulated largely at the perinucleolus (PN) stage, whereas almost no expression was found for the medium and long transcripts at theoogonia stage and moderate signals were detected at the PN stage. The differential expression of the three forms of transcripts suggested thatvarious transcripts might perform different roles during oogenesis of the kuruma prawn.