Differential display of mRNA was used to identify an upregulated gene in ovaries of artificially maturing Japanese eels (Anguilla japonica). Accordingly, mitochondrial (mt) cytochrome b, whose transcript levels increased from early to late vitellogenesis, was isolated, cloned and sequenced. Temporal trends in artificially maturing eels were compared with those in naturally and artificially maturing New Zealand eels (longfinned eel, Anguilla dieffenbachii; shortfinned eel, Anguilla australis) by Northern blot and in situ hybridization analysis to rule out any experimental artifacts. An increase in ovarian mt cytochrome b signals was seen when comparing immature and midvitellogenic longfinned eels, but not immature and early vitellogenic shortfinned eels, from the wild. Long-term captivity yielded reduced target mRNA levels, but abundance increased after hormonal induction of vitellogenesis. These results imply that the increase in mt cytochrome b mRNA levels during artificial maturation reflects natural development, although its onset appears to be brought forward during artificial maturation in the Japanese eel. It is suggested that increased mt cytochrome b mRNA levels result from both mitochondrial replication and increased transcription, and that they reflect the build-up of machinery for enhanced ATP-synthesis at some stage of oogenesis and/or early zygote development.