We report the first record of fecampiidan platyhelminths parasitic in tanaidacean crustaceans. Two fecampiidans (0.75 mm and 1.10 mm in length) were found in a female of Pseudotanais sp. (Pseudotanaidae; 1.75 mm in length) collected at 794 m depth off the southern coast of Japan, northwestern Pacific. Fresh individuals were yellow or light yellow, but completely faded in ethanol. In a maximum likelihood tree based on 28S rRNA sequences, the parasite was placed in a moderately-supported Fecampiidae clade, suggesting it is a member of Fecampiidae. The 28S sequence from the parasite was 25.0%, 32.6%, and 35.5% divergent in Kimura 2-parameter (K2P) distance from Fecampia cf. abyssicola, Kronborgia cf. amphipodicola, and Kronborgia isopodicola sequences, respectively.
