We have isolated and, by electrophoresis, using agarose slab gels in the presence of methylmercury hydroxide, analyzed the mitochondrial RNA content of the liver of rainbow trout. The RNAs corresponding to most of the mitochondrial DNA-encoded genes have been identified. Furthermore, among the transcription products we have also identified the nature of the RNA 8 previously described in human mitochondria, and detected a novel transcript that may represent the mRNA for ND6.