ABSTRACTThe aim of this study was to analyze the frequency of occurrence of individual stop codons and combinations of stop codons with adjacent upstream and downstream triplets in 264 bacterial and 1308 mammalian mitochondrial genomes. For the purpose of this analysis a novel program (Gene Triplet Analysis) was applied. The obtained results indicate that the standard stop codon UAA is the most frequently used one (48%) in both bacteria and mitochondria. In addition, our analysis revealed 30 nonstandard translation termination codons in mitochondria. The preferential nucleotides in all three positions adjacent to the stop codons in mammalian mitochondria are A and U.