The meiotic maturation of oocyte and spermatocyte in animals is controlled by the maturation promotionfactor (MPF), a complex of Cdc2 and cyclin B proteins. To better understand the mechanism ofoocyte and spermatocyte maturation in fish, the expression of cyclin B1 (CB1), B2 (CB2) and Cdc2 kinaseduring oogenesis and spermatogenesis in rainbow trout were examined at both the mRNA and proteinlevels. Quantitative real-time PCR analysis showed that the amount of CB1 and CB2 mRNA was greaterat previtellogenesis and late vitellogenesis stages, but less at early vitellogenesis stage and during earlyembryogenesis. Cdc2 mRNA was continuously present throughout the processes of oogenesis and earlyembryogenesis except for a decline at early vitellogenesis. In situ hybridization analysis indicated that CB1,CB2 and Cdc2 transcripts were present in oocytes of different developmental stages as well as in all spermatogeniccells except for spermatogonia. Immunohistochemical analysis revealed that CB1 protein wasabsent in vitellogenic oocytes, but present in young previtellogenic and mature oocytes. In contrast, CB2and Cdc2 proteins were present at all stages oocyte development. Similarly, CB2 and Cdc2 proteins werepresent throughout spermatogenesis, whereas CB1 protein was only detected in spermatogonia and spermatocytes,but not in spermatids. Thus, it appears that CB1, CB2 and Cdc2 transcripts have similar expressionpatterns during oogenesis and spermatogenesis, but CB1 protein varies in amount during these processes. These data suggest that CB1 may have a leading role in the regulation of meiotic maturation of oocytes andspermotocytes.