AbstractIn this study we present evidence of an ongoing speciation event in the Japanese Archipelago. The Kisaura nozakii species complex (Philopotamidae) is a group of a small trichopteran insects, and it has been reported that this species has large differences between populations in the copulatory organs (genitalia) of males. Therefore, we conducted molecular phylogenetic analyses based on the mtDNA COI region (658 bp) and genome-wide nDNA single nucleotide polymorphisms (SNPs) (16 254 loci) using the K. nozakii species complex and a sister-species, K. borealis. We also conducted principal component analyses (PCA) based on the forewing length and seven morphological characteristics of the male genitalia. It was revealed that the K. nozakii species complex is composed of multiple allopatrically differentiated genetic lineages. However, two genetic lineages were distributed sympatrically/parapatrically in the western area of the Japanese Archipelago, and the occurrence of ‘character displacement’ in the male genitalia was detected in this area. We consider that the evolutionary event in this species complex has high potential to become a model case for elucidating the processes of speciation and species diversification.