Objective To clone and analyze the whole sequence of mitochondrial cytochrome b (cyt b) gene of swordtail fish ( Xiphophorus helleri ).Methods\ Total DNA from the liver tissue of swordtail fish was extracted and a pair of specific primers were designed and processed for PCR amplification.Analyzed by agarose gel electrophoresis,the amplification products were purified and cloned into T vectors of pGEM\|T easy vector system.With transformants selected and recombinant plasmids extracted,the inserted gene was sequenced.Results\ The whole sequences of cyt b gene of swordtail fish were obtained.Conclusion\ Compared with sequences of cyt b genes in GenBank with BLAST,there were good homologies among swordtail fish and other fishes.The evolution tree built from the homology of cyt b genes of swordtail fish and other 13 kinds of fishes was consistent with the traditional taxonomic results.