The 16 776 bp mitochondrial DNA (mtDNA) of Crocodylus niloticus was sequenced by PCR methods. Comparing with the complete nucleotide sequences of the mitochondrial genome of other crocodiles, no variation on gene arrangement or quantity was found in the mtDNA of Crocodylus niloticus, however, the control region varied greatly. In addition to the analyses of complete sequences, partial sequences of 12S rRNA, 16S rRNA and Cyt b gene from other 13 crocodile species were combined and analyzed. We attempted to diagram phylogenetic relationships among crocodile species to provide convenience for following research.
The nucleotide sequence of the complete mitochondrial DNA （mtDNA） molecule of the salt-water crocodile （Crocodylus porosus） was determined in this article. The molecule is 16,917 base pairs Cop） in length, and codes for 22 tRNAs, 13 protein-coding genes, 2 rRNAs, as well as a control region （D-loop）, as is characteristic for mitochondrial genomes of other metazoans. The gene order conforms to that of other crocodilians sequenced, but the arrangement of some tRNA genes differs from other vertebrates. It shows that the gene order of crocodilians is remarkably conserved. In this study, the relationships among crocodilians were examined in the phylogenetic analysis based on the control conserved regions of 17 crocodilians. The results suggest that the gharial （Gavialis gangeticus） joins the false gharial （Tomistoma schlegelii） on a common branch, and then constitutes a sister group to traditional Crocodylidae. Thus, the result supports that G gangeticus belongs to Crocodylidae. The analyses also suggest that the African slender-shouted crocodile （Crocodylus cataphractus） can be treated as an isolated genus, and constitutes a sister group to Crocodylus.
