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Whether and where rainbow trout aryl hydrocarbon receptor nuclear translocator (rtARNT) isoforms are expressed in juvenile and adult tissues of rainbow trout is unknown. Using reverse transcriptase polymerase chain reaction (RT-PCR), expression of the rtARNT(b) isoform messenger RNA was identified in day 19 and 23 embryos, in day 27, 35, 39, and 42 sac fry, and in all adult tissues investigated. The rtARNT(a) isoform mRNA was expressed in all juvenile trout except day 42 sac fry and in all adult tissues except skeletal muscle. Western blot analysis and immunohistochemistry demonstrated that the rtARNT(b) protein was present in all juvenile trout and adult tissues investigated, except skeletal muscle, and was primarily localized to the nucleus. In contrast, rtARNT(a) protein was not detected at any developmental stage but was expressed in the adult gill. These results imply that rtARNT(b) is involved in signaling events at many developmental stages, while the functionality of the dominant negative rtARNT(a) is restricted.