‘Sekizaisou’ is a mulberry tree with drooping branches and unusual red-colored wood. It was discovered around in 1912 by a farmer on Okushiri Island, and propagules of the cultivar have been preserved by sequential vegetative propagation ever since. The leaves of ‘Sekizaisou’ had been used as sericultural feed since 1916 because farmers of the island believed in their positive effects on silkworm growth and silk cocoon quality. However, experimental data on the nutritional quality of the leaves has never been reported. To assess leaf quality as sericultural feed, silkworm larvae were reared on ‘Sekizaisou’ and ‘Shin-ichinose’ leaves. The digested matter of ‘Sekizaisou’ leaves seemed to be more efficiently retained in the silkworm pupae and cocoons compared to ‘Shin-ichinose’, though no positive effects on growth were observed. ‘Sekizaisou’ may have unique properties for future mulberry breeding on sericultural use.