The complete nucleotide sequence of the mitochondrial genome was determined for the fish tapeworm Diphyllobothrium latum. This genome is 13 608 bp in length and encodes 12 protein-coding genes (but lacks the atp8), 22 transfer RNA (tRNA) and 2 ribosomal RNA (rRNA) genes, corresponding to the gene complement found thus far in other flatworm mitochondrial (mt) DNAs. The gene arrangement of this pseudophyllidean cestode is the same as the 6 cyclophyllidean cestodes characterized to date, with only minor variation in structure among these other genomes; the relative position of trnS2 and trnL1 is switched in Hymenolepis diminuta. Phylogenetic analyses of the concatenated amino acid sequences for 12 protein-coding genes of all complete cestode mtDNAs confirmed taxonomic and previous phylogenetic assessments, with D. latum being a sister taxon to the cyclophyllideans. High nodal support and phylogenetic congruence between different methods suggest that mt genomes may be of utility in resolving ordinal relationships within the cestodes. All species of Diphyllobothrium infect fish-eating vertebrates, and D. latum commonly infects humans through the ingestion of raw, poorly cooked or pickled fish. The complete mitochondrial genome provides a wealth of genetic markers which could be useful for identifying different life-cycle stages and for investigating their population genetics, ecology and epidemiology.