A report on the complete mitochondrial genome of the trematode Odhner, 1911, its new definitive host from the Russian Far East, and unexpected phylogeny of Azygiidae within Digenea, as inferred from mitogenome sequences.

New data on the complete mitochondrial genome of (Azygiidae) were obtained by the next-generation sequencing (NGS) approach. The mitochondrial DNA (mtDNA) of had a length of 13 857 bp and included 12 protein-coding genes, two ribosomal genes, 22 transfer RNA genes, and two non-coding regions. The nucleotide sequences of the complete mitochondrial genomes of two specimens differed from each other by 0.12 ± 0.03%. Six of 12 protein-coding genes demonstrated intraspecific variation. The difference between the nucleotide sequences of the complete mitochondrial genomes of and was 26.95 ± 0.35%; the interspecific variation of protein-coding genes between and ranged from 20.5 ± 0.9% (1) to 30.7 ± 1.2% (5). The observed gene arrangement in the mtDNA sequence of was identical to that of Codon usage and amino acid frequencies were highly similar between and The results of phylogenetic analyses based on mtDNA protein-coding regions showed that is closely related to (belonging to the same suborder, Azygiida) that formed a distinct early-diverging branch relative to all other Digenea. A preliminary morphological analysis of paratypes of the two azygiid specimens studied showed visible morphological differences between them. The specimen extracted from Sakhalin taimen () was most similar to Thus, we here provide the first record of a new definitive host, , for and also molecular characteristics of the trematode specimens.