A new maximal circular code X 0 (MIT) with two permutated maximal circular codes X 1 (MIT) and X 2 (MIT) is identified in the protein coding genes of mitochondria. The three subsets of 20 trinucleotides X 0 (MIT)={ACA, ACC, ATA, ATC, CTA, CTC, GAA, GAC, GAT, GCA, GCC, GCT, GGA, GGC, GGT, GTA, GTC, GTT, TTA, TTC}, X 1 (MIT) and X 2 (MIT) are in frame 0 (reading frame), 1 and 2 respectively. X 1 (MIT) and X 2 (MIT) are deduced by one and two circular permutations of X 0 (MIT) respectively. The code X 0 (MIT) has four important properties: a length of the minimal window to automatically retrieve frame 0 which is equal to five nucleotides; an occurrence probability equal to 6.3×10 −5 ; a low frequency (12% in average) of misplaced trinucleotides in the shifted frames; and an occurrence of four types of nucleotides in the first and second trinucleotide sites but no nucleotide G in the third trinucleotide site. Several biological consequences are presented in the Discussion.