Varietal differences of nutritive values of mulberry leaves were examined with special emphasis placed on nitrogen and amino acid contents. In this connection, production efficiency of cocoon shell of silkworms, Bombyx mori L., was compared among several varieties both in the spring and late-autumn rearing seasons. In the spring season, Hikojiro was most excellent in terms of percentage of cocoon shell weight, digestibility and production efficiency of cocoon shell (PECS) in six domestic varieties under testing. In the late-autumn season, Shimanouchi showed the highest PECS. Among the introduced varieties, Kashmir 7, Kashmir 11, Pakistan 4, Pakistan 14 and Turkey 3 showed high PECS in spring, among which only three varieties, i.e. Kashmir 7, Kashmir 11 and Pakistan 4 indicated high PECS values in late-autumn. This result indicates that there are varietal differences in nutritive values of mulberry leaves. There were high correlations between PECS and nitrogen content of mulberry leaves, as well as between PECS and amino acid contents including methionine, histidine and threonine. The former two are restriction amino acids for the growth of silkworms, and the last amino acid is required for the synthesis of silk protein.