The complete mitochondrial genome of the Onchidium struma is important for general molecular and evolutionary studies in Onchidiidae.Via long and accurate polymerase chain reaction(LA-PCR),a complete mitochondrial genome sequence of the O.struma was determined and analyzed.The results indicated that the complete mitochondrial genome of O.struma a is circular molecule of 13 957 nucleotides.The gene content includes 22 tRNAs,2 rRNAs and 19 non-coding regions from 2 bp to 138 bp in size.Two protein genes and 4 tRNA genes are transcribed from L strand,and the other 11 protein genes and 18 tRNA genes are transcribed genes from H strand.All protein coding genes of the O.struma mitochondrial genome start with a typical ATN codon,except for ND2 with TTG.The usual TAA or TAG termination codons were found for eleven proteincoding genes.However,the COⅢ and Cytb genes have an incomplete termination codon(T).With respect to the secondary structures of 22 tRNA genes,the absence of DHU arm of tRNA^Ser was observed as in O.struma tRNA^Serand tRNA^Thr with 9 bases in anticodon loop was found,compared with typical 7 bases in others.The longest non-coding has secondary structures similar to the tRNA.Based on the deduced amino acid sequences of 13 proteincoding genes,the phylogenetic tree was constructed with Neighbor-joining(NJ)and Maximum parsimony(MP) methods and the phylogenetic relationships of six gastropoda were discussed．The results demonstrated that O.struma and Platevindex mortoni have closer phylogenetic relationship.