The previously unknown male genitalia of Phtheochroa unionana (Kennel, 1900) are described and illustrated. The species is dimorphic: one form is white with very faint yellow scales in the fascial areas and the other is white with distinct orange fasciae. The everted vesicae of the males do not show interspecific variation but are remarkably different from those of a closely related species. Apparently, the morphology of the everted vesica is a useful tool for species recognition in this genus. The conspecificity of the two forms of P. unionana is further corroborated by evidence from COI barcodes.